I am Kurose, a Fiora main that has been in challenger for 3-4 years now peaking Rank 10. This guide is just for showing bad matchups that you should dodge if you want to climb, and some itemization/rune guides.

When To Pick Fiora
You can generally pick Fiora in any situation if it's below Grandmasters because the people you will face aren't good enough to actually punish it. Ideally, you want to pick her as a counterpick to the top lane champion where she is allowed to freely scale and 1v9 the game such as into Dr. Mundo, Cho'gath, Garen, etc. If you see a ranged top laner, try to pick something else that goes well into it or you just might get bullied the enire game without being allowed to play.

How Good is Fiora?
Fiora is a very broken champion if you get to scale to 2-3 items, and no one on the enemy team has gotten abnormally fed. There is only a handful of champions that can actually resist her in the side lane, and only a few that can beat her in a 1v1 at even item spikes. She has some weaknesses, such as not being able to deal with poke/ranged champions that well, and she has no ability to push well early on, making her wave management really bad.

Reasons to pick up this champion
This champion is really good, and situationally 1v9 in a lot of comps. She has the ability to completely nullify comps such as double enchanters by being too much to deal with on the side lane, destroying tank matchups with hullbreaker on the side, and she has a lot of skill expression / mastery.

I will exclude many annoying matchups such as Teemo, Warwick, Volibear, etc, because I think that the lanes are playable, and that you can scale versus these champions. Reading a guide on how to play Fiora vs X champion will not teach you anything, it just leads to information overload.

To actually learn a matchup, you have to play against a good player on the champion you want to fight multiple times. For example, Fiora vs Aatrox has heavy misconceptions. The matchup is a skill based one, where it is easier to play on the Fiora side, and harder for the Aatrox side to play. Fiora will outscale and beat Aatrox later in the game, but Aatrox can win lane by playing with Q spacings at lvl 4-5 onwards and saving his dash to react and dodge the parry stun.

What to build first general rule -

Goredrinker - buy if you want a standard/ solid item on Fiora. This mythic is her best one, and is only outvalued by stride in niche situations such as vs some ranged top laners like akshan/kennen.

BOTRK - Fiora's strongest one item spike for damage. Falls off around 3-4 items, but offers the most damage/burst at one item. Makes you squishy, but you also do a lot of damage. Build when you want to all in and kill the opponent or as a transition from early vamp scepter rush.

Stridebreaker - Vs 3 - 4 ranged champs that can kite you. Goredrinker usually outvalues this because of the 10% omnivamp it gives.

Hullbreaker - Vs tank matchups or free matchups where you think you'll be able to farm plates off of them. This item being built first is also good into ranged AP champions such as kennen and akali who struggle to deal/clear the cannon minion, while giving you a lot of resistances to them.

All the items listed in this are options that Fiora has for building as a 2nd-5th item. I'll list some of my opinions on what each item is.

Death's Dance - Good item overall, I think it is her best item secondary, provides a lot of tankiness and dueling potential. Generally built as 2nd or 3rd item if built ever.

Essence Reaver - Decent item that is cheap, gives burst damage, and completely nullifies Fiora's mana issues in the late game, allowing you to be on the map permanently. Build this when you think you don't need to be tanky, and will be required to stay on the map a lot.

Randuins Omen - Build this as a third item after deaths dance if you want to be tanky into AD members of the enemy team, I only recommend this if it's something like jinx adc on enemy team with a strong AD jg + top laner.

Anathemas Chains - Decent third/fourth item option if you're planning to split, and want to just completely nullify the opposing top laner or jungler. This makes 2v1's much easier, as only one target is realistically dealing damage to you. You can also put this on the enemy mid laner if it's only one AP carry on the enemy team and stack armor.

Force of Nature - Best magic resist item in the game, I would recommend building this if there is double AP carries on the enemy team as it also synergizes with goredrinkers active healing with the bonus hp.

Maw of Malmortius - A decent option if you need the 50 AD to carry. Recommended to build this if you're up against something like Akali 2nd or 3rd.

Stoneplate Gargoyle - Generally a 4th/5th item, with the bruiser item nerfs to the amount of hp you get, the shielding is not that great, but it is still a good stat stick item that allows you to tank many members.