Each player chooses from among the permanents he or she controls an artifact, a creature, an enchantment, and a land, then sacrifices the rest.
10/4/2004 If you control a permanent with more than one type, you can choose that same permanent for more than one of the choices if you want to. This makes it possible to select an artifact creature as both your artifact and creature, and then select a land and thereby keep only two cards.
10/4/2004 If you control a permanent with more than one type, you can choose that permanent to be the representative of either type. This makes it possible to have more than one creature (or other permanent type) on the battlefield. For example, if you select a creature for your creature and an artifact creature for your artifact, you get to keep both of these creatures.
10/4/2004 The permanents to save are chosen on resolution. You do not choose them when casting the spell.
10/4/2004 The current player makes their choices before the other player. After both players make choices, all the sacrifices are done simultaneously.
2/1/2014 Each permanent not chosen by a player will be sacrificed. If a permanent has none of the listed types (for example, a Planeswalker with no other types), it can't be chosen.