Every deck needs mana, right? Because most of the spells in this deck are relatively cheap, you can get away with a lower manacurve than most Extended decks at the moment. I'd recommend somewhere between 20 to 22 total mana sources, of the ones that get positive reviews below.
Vault of Whispers, Seat of the Synod, Great Furnace: These are your basic starting points, and the real skeleton of the deck. Without them, Goblin Welder becomes significantly less useful. Your deck is primarily UB, so four of the first two is a definite requirement. You won't play much red aside from Welder himself, and I don't advise using more than two.
City of Brass: The ultimate mana fixer. A good choice for colored mana since by the time the damage points add up the game will already be over.
Painlands: Obviously as with any 3 colored deck, mana fixing lands are a requirement. Because the deck is mostly blue and black, Underground River becomes the manafixer of choice. with blue the predominant color, Shivan Reef is the second option. Sulfurous Springs should not be used.
Glimmervoid: Using Glimmervoid is a creative approach to manafixing, but in this case; creative is bad. In the early game you're going to have a very tight manacurve, and drawing a hand of one painland and one Glimmervoid can force a mulligan. Plus, the additional pain caused by City of Brass is unlikely to end up costing you the game anyway.
Chrome Mox: A powerful 0cc Mana accelerant and a color fixer. What's more, its even a good cheap artifact to feed to Goblin Welder. The downside to Mox isn't the lack of card advantage (since the game is unlikely to last long enough for that to be significant), but the lack of appropriate cards to imprint on the Mox.
Unsurprisingly enough, A deck with 20 to 22 total manasources isn't going to be regularly casting spells like Sundering Titan. Here's your cheat sheet for how to get 8 mana dudes into play on turn 2. Use at least ten total reanimation spells, but the recommended number is 11.
Goblin Welder: Don't ever use less than four. Welder is Instant-speed reanimation, 1 mana reanimation, and is great for doing broken stuff like swapping a pair of Sundering Titans into and out of play, much like Kiki Jiki, Mirror Breaker in Type 2 Tooth and Nail decks. Welder makes the entire deck tick, and playing without four of them is a waste of time.
Exhume: Some match ups (RDW comes to mind) a Goblin Welder won't stay in play very long. A 1/1 body is pretty easy to kill, and being Smotherable doesn't help. On the other hand, a 2 mana Sorcery is plenty fast, and if you only need to get one creature in play to win, it's almost as good. It also provides necessary consistency for games where you don't draw Goblin Welders.
Reanimate: Much like Exhume, this is your Goblin Welder backup. It has the side benefit of being able to steal something out of an opponent's Graveyard, and is thus quite tech when facing other Reanimation-based strategies. The life loss is unpleasant, since most of your threats cost 7 or more mana, but is hardly something to worry excessively about if you're getting a turn 2 Sundering Titan.
Show and Tell: It's 3 mana "reanimation" without requiring anything in your Graveyard. The drawback isn't very serious, since most other decks won't be playing threats of the same caliber as yours. On the other hand, it's significantly worse than even Exhume in the mirror game, and can't be used to get at threats you have already discarded. It's best to avoid playing maindecked Show and Tell, but it is an acceptable sideboard option.
Wanna win games? Use these cards.
Sundering Titan: The bread and butter threat of the deck. He's a 3 turn clock, making him too big to ignore, but his "leaves play" ability often makes him too dangerous to kill. The LD is great against any decks with basic lands, and if you're on the ropes, that 10 toughness makes him great at blocking even the largest of attackers and living to tell the tale. Using less than 4 Sundering Titans is a most grievous deckbuilding error, regardless of metagame.
Platinum Angel: Versus any deck that can't remove her (such as some versions of Affinity or Life), Platinum wins the game as soon as she hits play. Even against decks that can kill her, she can often buy a few precious turns for your other creatures to go lethal. Typically it's best to use two, allowing you to avoid drawing multiples (hopefully) but guaranteeing you can use Intuition to get one into the Graveyard when you need it.
Bosh, Iron Golem: A very powerful body and a stellar ability. The 4 mana to use hims is somewhat of a drag, but there comes times when you simply cannot manage to force an attacker through for damage, or the opponent plays the (formerly common) reanimator hoser of Ensnaring Bridge. Best used as a 1-of card.
Masticore: A common choice, chosen more for his ability to act as a discard outlet than his ability to ping. In a deck with adequate other discard sources, I'm forced to question the soundness of the card. Best used in multiples of 0, in my opinion.
Triskeleon: Masticore on crack. On the offensive side, you can still swing for 4 damage, but now you can remove counters for an extra 3 should you need it. More importantly however, is the ability to ping without sucking up mana. Keeping in mind that often it's using lands that will be feeding Goblin Welder, This is very important. In addition, a large number of the "trouble" creatures (Goblin Welder, Disciple of the Vault, Grim Lavamancer, etc.) are 1 toughness, meaning the Tricycle can often take down 3 threats singlehandedly. Best used in multiples of 1 or 2
Duplicant: Powerful removal meets a gigantic threat. Especially strong against other reanimation-based decks, where the creatures are both too big to kill otherwise, and will often come back for more when killed anyway. Duplicant solves both of these problems while giving you a gigantic threat in return. The downside is that versus a deck with small or no creatures, the natural two power makes him a very slow clock. Fortunately, you shouldn't ever be reanimating Duplicant in those matchups, so this is not likely to be an issue. As with Triskeleon, 1 or two Duplicants is the norm. *Author's Note, Duplicant and Triskeleon are usually an 'either or' card, playing both of them as a 2 of is not advised*.
Karn, Silver Golem: Typically relegated to the Sideboard, Karn is a dominating powerhouse versus Affinity, and the mirror match. His biggest asset is the ability to turn 0 CC artifacts (such as the lands) into 0 toughness creatures. This combos well with your Goblin Welders, since it allows you to kill a land, Weld out a threat returning that land to play, and then kill the land again.
We've got the giant win conditions, and the great reanimation spells, now all we need is a way to get stuff into the Graveyard. Mill yourself, discard it, tutor it straight into the Graveyard, here's your options.
Careful Study: The best way of getting stuff out of your hand. Any of the threats you have in hand are essentially as useful as blank cards that do nothing. Study draws you two new cards, and gets those blank ones into your Graveyard where they start doing something useful. In this deck, its even better than Thoughtcast, and a no brainer as a 4 of.
Hapless Researcher: Much along the lines of Careful Study, the Researcher provides you with a method of turning "blanked" cards into useful stuff. He can also chump block, which is a surprisingly important consideration. The other nice thing about Researcher is the Instant speed, letting you sacrifice him in response to an opposing Exhume, and getting around the possibility of your creature getting stolen by a Reanimate.
Intuition: The big pimp tutor. Moving those one and two ofs directly into your Graveyard, Intuition gets you exactly what you need. It also can find reanimation if you are in need of a way to reanimate your threats. For consistency's sake, it's best to use 4, despite the high casting cost.
Entomb: Entomb is banned in extended. Don't play it or you'll get disqualified.
Cabal Therapy: Mostly used as a "filler" card, Cabal Therapy is most useful in metagames with large amounts of combo or permission. That said, Cabal Therapy is quite a strong filler card, it's just not as good as Careful Study or Hapless Researcher.
You probably still have a few slots left in your deck still. Here're a few cards that don't fit conveniently into one of the 3 parts of your combo, but are still very strong.
Fire/Ice: A very powerful card. Often it will trade 2-1 with opposing threats. It can let you win a race, by tapping down something your opponent controls, AND as a bonus giving you another card. It's also a good card to imprint on a Chrome Mox, giving you access to both red and blue mana. Definitely a good choice.
Vampiric Tutor: Another consistency booster. Vampiric Tutor gets you whatever card you need for the low price of 1 black mana. It's not a good idea to run a full playset, because often the tempo loss from casting multiple copies can be disastrous.
Spells to Avoid
Every netdeck will have people who don't fully understand it and advocate using bad spells in it. Here's a few spells to avoid.
Mindslaver: Unlike in Vintage, you are not going to be able to take good advantage of Mindslaver here. The mana curve is fairly tight with artifact lands being the primary items you are feeding to Welder, so getting that 4 mana activation can be difficult. Even worse, quite often, Mindslavering an opponent is a "win more" process, as most of the decks that really get hosed badly by Mindslaver, also get hosed even worse by Sundering Titan.
Trinisphere: Possibly a very useful sideboard card for dealing with the likes of Aluren or Mind's Desire, The important thing to remember about Trinisphere, is that every single spell in your deck (that you actually plan on casting) costs 3 mana or less.
Stitch Together: You already have plenty of reanimation. Running more than the stuff mentioned above just clogs up your hands, and makes it harder to get stuff into the Graveyard when needed.
So how do you plan on winning game 2? Here's a few methods.
Overload: It's not quite Oxidize but it's pretty darn good. Great at dealing with everything from Isochron Scepter and Sapphire Medallion to Arcbound Ravager and Cursed Scroll, all for a mere 1 red mana. It should be noted that this is almost strictly superior to Shattering Pulse which will almost never blow up anything more expensive than two mana anyway, and will only rarely be bought back.
Engineered Plague: Aside from the obvious uses against tribal decks like Goblins and Elves!, Plague comes in strong versus Aluren (beast), Cephalife (Soldiers) and random stuff from left field like opposition or anything relying on Decree of Justice to win. Typically played as a 3 or a 4 of in the sideboard
Karn: Mentioned above, Karn is a strong defensive card, with an ability to dominate the mirror and affinity matchups. Used typically as a 1 of that gets fetched via [card]Intuition[card[ and then discarded.
False Cure: Umm yeah. This is *some* good against Life. It's not so hot if they are keeping/bringing Orim's Chant in for game two, but they probably aren't. The cure only gets 1 spot (if any) in the sideboard, and is Vampiric Tutored for. As a side note, this is also very strong against versions of Mind's Desire packing Tendrils of Agony instead of the conventional Brain Freeze.
Stifle: One of those all around useful cards. Great for dealing with everything from Gilded Drake to the imprint trigger on Isochron Scepter, to stopping a Goblin Welder, for a single turn, to the storm trigger on Desire. Use as many as you have room for.
Crumbling Sanctuary: Another hoser for dealing with Life. You're never going to kill your opponent, but with a sanctuary in play, you can end up milling your opponent to the death rather easily.
Trinsphere: Another of the combo-hosers, this time aimed at Desire and Aluren. Both decks have a very difficult time going off in the face of Trinisphere. It also more than doubles the cost of using an Isochron Scepter, as well as moderately useful against any deck with a very low manacurve such as RDW.
Chill: How much RDW do you expect to face? Very Little/None, Little, Some, Lots, Really Friggin Lots? That answers the question of 0,1,2,3 or 4 chills respectively.
Gilded Drake: It costs two mana, and steals you an Akroma. Better still, it comes into play when your opponent casts Exhume.
Petradon: It's like Sundering Titan, but it can't be welded, the lands return when it dies, and it can hit nonbasic lands. I don't like them, but Gabe Walls does, and when he likes a card, people start playing it.
Netdeck Netdeck Netdeck
Here's a few recent PTQ t8 decklists for you to analyze. They are not perfect - with mistakes as subtle as Shattering Pulse sideboarded in preference to Overload, or as blatant as only 2Sundering Titans - but you should be able to spot what is good and bad about each deck, and pick and choose what you like when you build your own Teen Titans deck.