I'm a novice to League, silver 3, but I'm also a Ph.D. in chemistry and love looking for creative ways of playing the game. My builds are always wild, and many won't be as effective as the obvious builds, but if you're looking to shake up your gameplay and have a fun time then I highly recommend trying these out :)

I haven't played League since 2013 when Zac first came out. A lot has changed since then, including the introduction of AP Twitch. I played with guides for both AP and AD only twitch and noticed big holes in each of the builds.

I noticed AP Twitch has a huge early laning advantage once you purchase blasting wand and dark seal. He effectively clears an entire wave with W, three hits, and E. Popping out of Q on a champion with hail of blades and W is incredibly effective to quickly assassinate someone early if their support leaves or if they have little CC. However, as the game progresses Twitch becomes squishier and squishier compared to fellow assassins, and can barely get 6 stacks before being killed. Also, in a team fight Twitch does almost no DPS until he gets max stacks on a champ and can land an E. Even then, after using E and R Twitch is very weak in team fights and essentially needs to run away until at least the E refreshes.
TLDR - AP Twitch is OP in lane/early, but lacks the sustained DPS to effectively team fight.

AD Twitch on the other hand is excellent late game after building KS/ISB, Runaan's, and IE. However, almost every single other bottom lane champion has better wave clear, trade, and all ins early on. This means it's very difficult to snowball early, and so AD Twitch relies on an excellent support/jungle and perfect last-hit mechanics under tower (which rely on his mana draining W and E abilities). AD Twitch is simply not as powerful as other bot champs in lane.

This is when I began experimenting with starting as AP twitch to evenly match bot champs in lane, and then transitioning to AD Twitch as team fights and objectives become more important. I made many mistakes, mostly either building AP for too long or wasting money on crit hit chance early, etc. This guide and build represents many hours of gameplay and several hours of measuring DPS at each stage of the build in the training tool. I think what I've developed here is a solid enough build for you to win many games, but might not have the win rate of either AP or AD Twitch. We'll just have to wait and see how it develops ;)

It's pretty simple - this build has low base AD, so money spent early on crit chance is wasted. If you want to play twitch full AD, then build KS or ISB, Runaan's, and IE or BT as the core. The crit chance and damage balloons quickly and Runaan's extra bolts receive those crit damage/chance bonuses as well.

Nashor's is key to build early, because with blasting wand and dark seal in lane Twitch's passive is essentially an ignite doing 101 true damage at full stack, and ripping through minion waves with E. Also, the on-hit magic damage applies to all of Runaan's bolts, effectively tripling the magic damage in early team fights.

After building nashor's if you ever have 1300 gold you should buy noonquiver. Having this AD boost before finishing Runaan's means that Runaan's crit hit chance is not wasted. This is ideal, but if you are not doing well then I'd recommend skipping this and rushing Runaan's. This is because you'll gain a huge boost in DPS in teamfights, hopefully helping your team take an important objective or tower/inhibitor.

I tested using blade of the ruined king as the third item. I thought it would do more damage, especially late game, because it scales with health. However, when I compared it's DPS and healing to blood thirster it was clear that BT won out. You might ask, why immortal shieldbow instead of kraken slayer? The DPS is surprisingly similar, and Twitch has no escape whatsoever, so the shield is critical. When you have these three core items you should be very very powerful.
This part is important - when building ISB make sure to buy Noonquiver, then vamp scepter, then cloak of agility. Both the lifesteal and crit hit chance are worth much less without bonus AD.

Let's talk 4th items -

That being said, I've found that after the first three items someone on the opposing team is either fed and targeting you as a squishy Twitch, or your team is about to win the game and it doesn't really matter what items you buy next. If someone on the opposing team is fed be sure to use the 4th item slot to buy a defensive item like wit's end (which applies to each runaan's bolt, tripling it's magic damage) or zonya's. This has saved me a couple of games, and both items provide twitch advantages since he benefits from AP and AD.

Overall, the 4th and 5th item for this build is completely game dependent, but remember to take advantage of Runaan's Hurricane unless they're for defensive purposes. Black Cleaver is excellent for applying armor penetration, mortal reminder to reduce healing, or Rabadons for more passive and E damage to break down low HP teams.

Honestly, other Twitch guides or Youtube videos are probably better for advice on gameplay. One piece of advice I have is that Doran's Ring is critical for the mana regen early on. Without this Twitch can't both trade/wave clear and go all in when the jungle comes by. Also, Dark Seal is so cheap for the snowballing potential it possesses.

Random specifics to this build - since the escape capabilities of Twitch's Q got nerfed he is incredibly vulnerable to hard assassin dives like from Zed or Nocturne. Do not use this build when playing against champs like those. Instead build all AD with ISB, or all AP with Zhonya's. This also means that the general rule that ADCs only hit the champ right in front of them is even more important for Twitch.

Maybe this is talked about in other AP guides, but you have a big power spike after purchasing Blasting wand and dark seal. If you're roughly even with your opponent you can probably let them push near tower, and then dive using Q, W, 3x attack on ADC, E to get a quick double kill (if your support is involved too of course).

The same power boost that comes from blasting wand early applies to wave clear. If you land your W to hit all minions, wait a couple seconds for the melee minions to get weak, and then E you can clear the whole wave at once from ~1000 range. I've been in situations where my support roamed and I could STILL push waves faster then the enemy ADC/support combined. This usually drains mana rapidly, so it's key to hit a champ with the W to get the presence of mind mana burst.

All of this together means you should ALWAYS prioritize blasting wand -> dark seal -> dagger -> boots on your first back. Whenever I have ~750 gold and get dark seal and boots, I have a harder time than if I get dark seal + dagger, or even just dark seal and wait until I have enough for blasting wand.

In bot lane especially you're probably less powerful than your opponent's support/ADC until you get blasting wand dark seal. This means I'd recommend letting yourself be pushed close to tower to safely farm until you have the 1200 gold for both or at least 850 for BW. Obviously, if the enemy is playing recklessly, or if you have a gank or great opportunity from your support to get a kill then take it. But do not play aggressive very early bot lane.

And now to reverse myself for mid lane! Almost every mid lane champ relies on combinations of the abilities to CC or damage stack. This is not the case for twitch. At level 1 twitch is stronger than almost any other mid lane champ, especially melee ones. You should play extremely aggressively to prevent the enemy from getting all 6 first CS AND deal some damage to them. Once you both hit level 2 then they probably have the advantage, but they should have lower health and/or mana than you from your constant harassment. The very beginning of the game is crucial for mid lane twitch.