I am a diamond player and enjoy playing mf. This guide is not on a professional level but will get you through enough.
Let's dive into the build options. First of all i hardly ever play crit. It's not weak but i just enjoy lethality or hybrid builds more than crit builds. And in my opinion they are stronger and easier to play. So if you are new to the champion or just want to gain ranks with it i highly reccomend playing hybrid or lethality. Because even if you fall behind you can just farm a little bit and get to the third item and deal massive damage with just your ultimate. Remember your ultimate is the strongest thing you've got but its also when you are the most vulnerable.So be careful with that. Before team fights dont just waste your ult. Try to wait a little bit to let your teammates cc someone from the enemy team. And even if its just an enemy and he is guaranteed to die to your ult then just use it. Dont wait for the perfect 5 men ult. I know its satisfying but if you are not in low elo it is unlikely to happen. Unless you have some champs like orianna or rell in your team.

My apologies for this sloppy guide. I will try to wrap it up asap.
To finish this guide i'll tell you the most cruical tips for mf.
*Select pta if you are new or want early dominance.
*Your early damage is most likely more than the opposing adc. Use it to your advantage. But keep in mind that support picks also matter. If the enemy support is like nami or velkoz that has high burst early on you gotta back off a little bit first and bounce q's on low health minions.
*Speaking of bouncing q's dont just focus on low health minions. A normal q would deal just as fine damage. But crit q deals some sweet damage. So try to land those especially in hard match ups.
*Dont wait for the perfect 5-man ult.
*Learn the mechanic of your w movement speed. To summarize when pressing w it fully restores the movement speed passive but if you even get a damage from a minion it'll reset. Also keep in mind that movement speed restores when you dont get damage.
*Remember you have no dashes but your insane movement speed combined with youmuus will outrun almost any other champion. Use this to your advantage and have some space between you and the enemy team.
*Btw you can bounce q's even if you dont have vision on them. You can use this mechanic to check if the bush is clear or not. Or just poke with this unexpected bounce.(most players dont know this mechanic)
*Dont spam your e ability. It consumes A TON OF mana. It'll drain your mana in laning phase. Just use it to slow your enemies before you engage or all in trade.
*Proc your PTA with this combo : auto attack / q / auto attack. This will win almost any short trades. If you are able to just press w and back off then wait for your q.

Now for the build tips ;
*Max w if you go lethal tempo it'll give you a huge attack speed alongside the sweet sweet movement speed.
*Again highly reccomend the PTA rune. Is just bread and butter with mf.
*If you have poke support like velkoz,xerath you can go arcane comet and buy tear. So you can spam e or q to proc comet on enemies and make the laning phase hell for them. Dont reccomend going comet against enchanter supports with shield abilities.


And thats all for now, i will be updating this guide in like 2 weeks cause i have an important exam upcoming. See ya !