Keep your passive up by poking enemies with Q(or farming if you're not particularly good at positioning yourself to q them and not take harass, honestly just throw it wherever as long as it hits to get the lower CD to keep your passive up) here and there then look for a trade if there's kill potential, which you kinda just learn from the matchups and it'll depend on your support. If there's no kill potential, just go get tear ASAP, the sooner it turns into Muramana, the better it is. I prefer Triforce 9 times out of 10 to IBG, but IBG's still decent if you want to go that route, and Bork is like, your best friend. Make sure you use W to get the free fervor stack and passive stack when you're going in, and hit as many people with it as you can. IE: Leona E-Q's ADC, W Leona and the ADC for 20% atk speed right off the bat, Leo attacks faster, and if you hit their supp, too, coolio. Once your Muramana is stacked, keep in mind it applies to autoattacks AND single target spells, meaning actually landing E on an enemy champion in a 1v1 situation or where you're catching someone out of position will give you a lot of damage. Also, against stealth champs like Vayne or Twitch when you know their general location, you can E and then Q wherever your E's bolt goes (since it'll go to them), which can really help in those two matchups as far as keeping DPS and passive up in a fight against them goes.
But as far as just playing him goes, it's mostly about how to play him against the different matchups in laning phase(farming, trading, or all-inning) and how to play him with your specific team comp(engage, siege, etc), and once you know that you can pick him into just about any matchup and/or teamcomp and do pretty well.