FIRST OFF I must ask you too read then entire Guide if you wish to fulfill the max potential that AP Shaco has to offer Now to begin the guide.

AP Shaco is a very fun and VIABLE way to play Shaco. It is overpowered but is reliant on you NOT dying. All around fun but difficult to learn.

Pros & Cons
Fun, Overpowered and not hard to play
Reliant on items (Like Mejai's Soulstealer)
Great solo top
Hard to recover from a bad start
Not reliant on Minion farm
Needs a lot of kills/Assists to be it's best

Early game: At level 1 Ap Shaco is greatly underpowered and isn't able to 1v1 most champions, wait until you have a Blasting Wand ( and preferably a Mejai's to get early stacks) before trying to assassinate anyone. On another note If by chance your team doesn't have a Jungler, take a buff at level 1 with Jack in the Box, To do this, place a jack in the box directly where the Monster with the buff will be standing at around 1:05, Then keep placing a Jack in the box every time the cooldown is up. Once the monster spawns wait for it to be feared then attack its back once it is attacking a Jack in the box, this should effectivly kill the monster and almost kill one of the little creeps next to him.
Mid game: Around this time you should have your boots, your Rabadon's Deathcap, and Mejai's Soulstealer(which should have a few souls by now.) This is one of the best times to become the monster that is AP Shaco, teamfights can be easily won with Jack in the box and your Hallucinate demolishing groups of champions, and picking off kills with Two Shiv Poison. Getting your Mejai's Soulstealer stacked up will be easy with group fights in this stage, this time in the game can make or break you.
Late game: At this point in the game you should be fully or nearly fully built, Now depending on whether or not you got your needed Mejai's Soulstealer stacks at this point can mean a lot of things. IF you DID get the stacks you need and are fully built you should be able to kill almost anyone on the enemy team with your Hallucinate off cooldown, and become the unstoppable force that is AP Shaco. IF you did NOT get the needed stacks for your mejai's stay back and get assists or kills if you can, but focus on living at all times, you should be able to come back easy depending on how the game is going.

Possible alternates
Clarity/Ignite is a great way to get quick kills and staying alive, But starting off they can be tough to play
Flash/Clarity is another good way to play, it is a more cautious and can help starter Ap Shaco players get into the rhythm of things.
Teleport/Ignite can also be an effective way to play, but It seems under-powered and mana hungry.

Skill priority:
Hallucinate/Two-Shiv Poison/Jack in the box/Deceive
Hallucinate: Summons a clone of Shaco and can be moved around and commanded to attack by holding Alt then Right clicking. This clone has very low Health and when it dies it explodes doing massive damage based on your Ability Power, This can be used to get kills on champions behind a tower (The tower kills the clone while the clone is trying to attack the enemy champion), and the clone can be used to greatly change the tides of a teamfight, effectively taking down half of the enemy teams Health.
Two-Shiv Poison: This is going to be your main offensive attack, can do massive damage at great range (taking down a great chunk of a single enemy champ when mostly built) and has the great slow utility, all together an OP move.
Jack in the box: A great ability used to both kill enemies and save allies, this ability does great damage late game and can save you with its AoE fear.
Deceive: Not very important to rank up fast, just one point early and it can be used as an escape spell the rest of the game. (Also note that the teleport goes slightly further than the clickable parameters)

I would recommend following the items in the Guide exactly but you may mix-match some items to your liking and still get what you want out of AP Shaco.
Possible Changes:
1: Some people may want to switch out Zhonya's Hourglass with a Banshee's Veil or another great MR item, But this is not recommended because of the great potential of Zhonya's Hourglass.
2: Another change some might like would be swapping out your Mejai's Soulstealer With another Ability power item like a Deathfire Grasp, This is a much safer route and could be used by someone new to AP Shaco (Note the AP lose can be counter acted by the DFG, it is purely a matter of preference)
3: A More courageous item route would be replacing your Guardian Angel for a Deathfire Grasp and your Zhonya's Hourglass with a Morello's Evil Tome. This build would be for a more experienced AP Shaco player, and would put out a lot more damage at the cost of defense and survivability.