The attach ability for 2 islands, on neurok stealthsuit, can you use that ability only if you've already cast it and it's on the battlefield? Or can I have it in my hand and as a result of someone targeting my creature, can I then tap 2 islands in response and attach neurok stealthsuit to it directly from my hand, essentially countering the spell targeting my creature
You can activate Neurok Stealthsuit's second ability only if Neurok Stealthsuit is on the battlefield under your control (C.R. 602.2), not if Neurok Stealthsuit is anywhere else, including in your hand. That's because that ability works only on the battlefield because it could work there and doesn't mention another zone where it works (C.R. 113.6). This applies to Neurok Stealthsuit's second ability just as it does to its equip ability, which you likewise can't activate from your hand even if you can activate equip abilities any time you could cast an instant (review C.R. 702.6a).
Neurok stealthsuit