Not Updated For Current Season
Rally: A better heal than Heal, plus it makes your crossbow bolts bigger.
Cleanse: It's absolutely essential for escaping ganks during those times when Ambush isn't up, for pulling a triple kill out of a mid-Spray and Pray stunlock (if you're dying, you're sure as hell taking people with you), and for extra insurance against smarties who invest in anti-stealth items.
Step 1: Use your gold wisely, and buy Doran's Shield and a health potion. This, combined with Rally, gives Twitch some excellent early-game survivability.
Step 2: You should be able to harass (and likely kill) your opponent long enough to save up for two Avarice Blades and level 1 boots (Boots of Speed). Avarice Blades mean bigger crossbow bolts AND more gold, which will speed you along to the all-important Infinity Edge, where the real bloodbath begins.
Step 3: Sell Doran's Shield, get your Infinity Edge, and pick up a Vampiric Scepter if you've got enough leftover gold. The Infinity Edge makes your crits bigger and badder. The lifesteal keeps you healthy, and lets you stay up just a little bit longer so that you can pop Expunge and take four people down.
Step 4: Mercury's Treads or Ninja Tabi, depending on what sort of protection you need more of. I usually go with Treads, because CC reduction is wonderful. Some people are probably gonna whine about this being so far down on the list, but Twitch don't need to run, because he only gets into fights he can win (thanks to Ambush, good map awareness, and all those big crossbow bolts of death).
Step 5: Build a Bloodthirster. At this point, you can do whatever you want -- if the game lasts this long, I usually fill my last slot with a Black Cleaver, and then replace the Avarice Blades with a Phantom Dancer and a Last Whisper or Tiamat. You won't even need to push buttons anymore; you'll just right-click to kill, with optional "Push E and/or R to Win."