Play safely. Play smartly. Play passive. Let the others make the first move and attack when they don't expect it. That's how you play Fiora. She's OP, YES! But don't go agressive, you'll do mistakes and you'll lose your advantage. Once you're sure you can kill your enemy easily, have fun! But don't forget the jungler...Put wards! and have map awareness!

If you're laning against a range such as Jayce, Nidalee, Quinn & Valor or big pokers like Olaf and Gangplank you should max your W first.

If your against a top with a cc such as Xin Zhao or Pantheon wait 'till they knocked up/stunned you before triggering your E.

Play passive.

Don't try to 1v1 your top, just keep farming and let the other engage the fight. When they do, judge if you're stronger than him, then kill him, but don't be too greedy. Minions are op and can give the advantage to your enemy. If he's more op than you, just back and towerhug until you get farmed and op.

Be sure to always put a ward.

-Boots: I like Mercury's for the tenacity and it gives better chance in teamfights with all the cc around, but against an ad top you can take Ninja tabi and have more fun killing him and get out of the fight full life. The Berserker's are also an interesting possibility depending of your playing.
The Furor enchantment with the Frozen mallet gives no chance of escape to your enemies.

-Black Cleaver for high damage early game.

-Frozen mallet will allow you to get kills and get more fed and tanky. You can chase and cath up your prey easily.

-Ravenous Hydra is just op on Fiora.

-Ruined king will do tons of damage on tanky people since you get it later. And even against squishies, you'll just heal yourself as f*ck

-Guardian Angels: It's hard to kill fiora once with the lifesteal she gets, try it twice with more resists.

Your Quintessences of Lifesteal make all the difference. With Ruined king, Ravenous Hydra and your passive, you're healing yourself too much, you won't notice you're gettinf focused in teamfight.

Avoid CC and don't be the one engaging or getting engaged. Jump on squishies like adc and mage depending the one who is more accessible or fed. It's not because you're fed that you're invincible. Your team is your strength. In fights, your lifesteal will deny zone damages and your Ravenous Hydra will just get the **** out of the enemy team. Don't forget to activate it!

1v1 or 1v2 or possible late game. Just be sure they are not both fed. In those situations, if you don't activate your Ravenous Hydra AND your Ruined King: you're dead. I recommend to activate the Ravenous hydra early fight for more sustain, and your Ruined King late fight to come back and finish the fight.

If jungling your build will cost a little more because you have to buy Wriggle's Lantern and sell it later.

I recommend to gank lanes where your allies have CC, since you have none. Your high damage will do major difference in ganks and your Q will allow you to go in quickly.