Hi this is supposed to become an easy and comprehensible Build on Fizz. Nothing extreme complicate. Hope u enjoy it and rate it.
I use it this way and i am surprised how good it works.
Sure I don't win every game... But 80% of the few games I lose are not because of my build.
Every build got his weaknesses I will write a bit about it later.
I am really sorry if my English is not the best is some chapters or if i wrote something wrong. I am an Italian who lives in Germany so pls. be clement.

Pros:
+very good DPS
+enough sustain to survive teamfights
+surprising DMG in melee combat
+nice DOT with your "W"
+still a good burst damage
+good versus the most midchamps

Cons:
-not the highest possible burst damage
-not so good against a few champions
(for example: Katarina she can escape easy or Vladimir so much sustain and an escape skill)

Red: (+0,95 Mag.Penetration)
Yellow: (+1,41 Armor)
Blue: (+1,2 AP)
Quints: (+4,95 AP)

These are my standard AP Runes. The Seals maybe could be Greater Seal of Replenishment but I Pic for some sustain. U will like them when the Jungler comes and in earlygame they are very helpful. My build focuses on the "W" and "E" skill. You will get in melee combat, where the armor will be needed.

Look on top. I go for AP-Dmg and some sustain. 21/9/0
I don't think that 9/21/0 or 21/0/9 would work better, because you would become squishy or loose dmg.
Optionaly you can change the 2 Points on to

I reccomend and .
Optionally you could use and (I don't like ghost at all because fizz don't need it... He ignore unit collisions with his passive.)
or and

You start with "E"(recommended) or "W" and invest one point on your "Q" on lvl.3. After that you max out your "E" and "W" constantly. Don't forget to max out your ulty when you can!

In earlygame u try to kill your laning enemy on level 3.
You dash with your and begin your autoattacks if necessary you evade to stuns or chase your enemy with your . Maybe you decide to dash with and Chase with . You got to see how it works better for you. Don't forget to activate your for the surprising extra damage in your attacks.

Later you initiate the fight with your . If the skillshot succeed your enemy has no chance to escape. The rest of the fight is similar to the earlygame. In teamfight try to hit a squishy target with your . If you nab an enemy carry you can easily take him/her down.
Remember: your main dmg come from your and your . You should be careful about when to into the fight and you must get a feeling for the moment the enemy start to focus you, so you can away in time to look who to focus next.

I start with a simple .
After that I buy
Often I must read in the chat things like: "look @ Fizz! troll item!"
This happens in the first 10 minutes of the match, while I build .
Right in the moment it is done your laning enemy and everyone who was skeptical will be like: WTF!!!
This nice little Item will boost your earlygame and midgame DPS so insanely. The Attackspeed stacks your so hard! In the same time you steal the enemy's Magic Resistance and you deal extradmg because of the passive and the extra AP!
Pls. don't run into the enemy like "Now I am the Boss!" remember you are still squishy! You can try to surprise your enemy. If he/she is not guardedly it will work.
Now finish your boots!
The necessary sustain to survive teamfights now come from in the same time it solves your Mana issues!
The next item should be Now you get another huge Dmg boost. The enemy will not ignore you... So you have to see if you are able to dodge their attacks. If you get focused often try getting out of teamfights earlier or getting into teamfights later. If nothing helps it is time for a . More sustain and an extra life every 5 Minutes. If you don't need it buy first.
This is my standard build. It can be altered with a few items. I did not test them all yet but a few of them are real alternatives to my standard items. Here they are:
Maybe an alternative to .
Maybe an alternative to too.
Another alternative to .
I prefer because it gives Mana and more HP. You don't need the slow of Rylai's because you already have one with your "E" !
If you rock the game this is the alternative to .

Try it and leave some feedback!

Like I wrote on top, this is not supposed to become something complicated.
Try it leave some feedback pls.
Thank you for reading!