Always try to hit your passive in trades. Use AA-Resets and block important spells of your opponent with your w. You can either predict them (if you know how your enemy trades) or you can react to skillshots or other spells with some casting time. Don't trade if your w is on cooldown. You can poke enemies using your q (try to hit passive).

Use your R early in a fight, but don't use it too early or the enemy won't fight. Use flash to proc vitals of your R that are hard to reach. Stormraider's Surge gives you a speedup so your enemy can't escape normally. You are faster than most champs in LoL with this build/masteries. Try to stick on your target and use your E to slow it. Don't use your W at chasing, it has a long animation that can be dodged easily. Make use of minions/monster at chasing/escaping. If you hit your Q on a minion/monster, the cooldown is reduced.

You can reset AAs with your q, e and the hydra active. Your Combo is:
Q -> AA -> E -> AA -> Hydra -> AA

Resets are used to cancel the animation of AAs (Auto-Attacks). You cancel the animation but the damage stays. This is the most important point in fighting with Fiora. You can use AA-resets for farming too.

Black cleaver is a must for Fiora. You get a speedup, some health, attack damage and penetration. All stats that Fiora wants.