You want to get a "Dorans Ring" first. Than you are gonna wanna get "Boots of Speed". Follow up with another "Dorans Ring" for the early game AP and HP. Then finish your "Sorcerers shoes".The "Sheen" is gonna be for a "Lich Bane" later. Then get a "Rabadons Deathcap" so you can have early game high AP. Finish your "Lich Bane" with the "Sheen".The "Giants Belt" is for Fizz being so squishy so u have mroe durability to your laning enemy or ganks in the future of the match. You are going to want to Sell a "Dorans Ring" and get a "Zhonyas Hourglass" for your Fizz inisiations to start a team fight and when you get low you can use thee passive. Use the "Giants Belt" in your "Rylais Crystal Scepter" mainly for the 500 HP to also help with the inisiations and keep you alive. Sell thee other "Dorans Ring" and get another "Rabadons Deathcap" so that you will have massive Burst Dmg against other squishy enemys such as Ashe , Teemo , Tristana , and many more. That concludes the step by step guide for the build to clarify what to do.

PROS
He has massive burst damage
His ultimate can hit all 5 enemy champions
He can get away by jumping thro skinny walls
He is a good dualist against AD Carrys and squishy APS
CONS
Very squishy
A little hard to farm when low mana
Cant fight tanks with alot of Magic resists
Semi-Good on Twisted Treeline