Okay, Smolder is FINALLY out, and boy, oh boy, do I have a great guide for you. Are you stuck in Low Elo? Play this champ.
Think of Smolder like Ezreal; you are more of an ability-based champ than an auto attacker. Smolder also has an infinite scaling mechanic; similar to Veigars, hit an enemy champion with your abilities, and you'll gain a stack, or you can kill a minion with Q to gain stacks. As you gain stacks, your Q evolves, first gaining splash damage, then adding a few fireballs that shoot out behind the target, and finally giving your Q a mini elder dragon buff. As well, all your abilities gain more damage as you gain more stacks. Not only is Smolder an ability-based champion like Ezreal, but he also builds similarly to Ezreal. You probably want to build Manamune, but it isn't 100% necessary if you decide to play crit. For a crit build, you'll need Essence Reaver and Navori Quickblades (fun fact Navori's passive actually procs on your Q, so Q will lower Q's cooldown); Bloodthirster is nice to have as well. On the other hand, you can build Trinity Force and Manamune. Personally, I prefer Navori as it feels way better. There is also a new AP build that has you just burn enemies down, with the first item being, Shojin, into Riftmaker and Liandrys, Rapidfirecannon and a 5th item of your choice.
Now aside from infinite scaling, Smolder is easy to play due to him not having any real combos. The only things worth noting are that he is able to Q flash or W flash (Q will come out from the new location, but W will not); you can NOT R flash to change the direction of your R. So don't waste your flash trying to do this. You also aren't able to weave autos with Smolder, so if you press Q and auto, it is just the normal slow wait to auto-attack. So you'll want to auto-attack, Q, and then start kiting while waiting for your next auto or ability. The final thing worth noting for Smolder "combos" is that when you use your E (the flying ability), if you press any other ability, it will cancel your flight, so if you're in a fight, your best bet is to use your other abilities before you use E.
The single most important thing for Smolder is his stacks. You'll need to farm effectively with your Q in order to gain stacks, and once you hit the first breakpoint of 25 stacks, it becomes much easier. Auto each ranged minion once or twice, and then Q the highest health one, and the AOE damage will get you 3 stacks easily. The same can be said about melee minions, or you could even be ballsy and Q the cannon to try and get the whole wave. The minions will have to be grouped up weirdly close, though. Once you hit 125 stacks, you shoot a few fireballs behind the target as well, so hitting the melee or cannon minions will also kill the ranged now. Past 225 stacks, you're golden. You will easily explode enemies. The best way to get your stacks though, is definitely from fighting. With the bot lane being a duo lane, you stack at double the speed, so taking fights more often is well worth your time. Even if you have to die once or twice.
Pro tip for playing against Smolder, if you're wondering how many stacks he has and can't click on the champ due to being in the heat of battle. You can actually look at his horns, the bigger and brighter the more stacks.
Another great move to utilize is using Smolders E to both dodge CC and chase down enemies. If you hit the enemy with your W, they will be slowed, and then you can easily hunt after them with your E, as it gives you a ton of move speed and the ability to go over terrain. Another use case for E is that if an enemy has a spell shield, your E will break it and continue to deal damage after breaking it! As well, Smolder's Q actually deals on-hit effects so items like Blade of the Ruined King are good buys.
Other than that, Smolder is an incredibly easy and powerful champion, and anyone can dominate the rift with this champ. Take fights often, build lethality, and get those stacks! Good luck and have fun!!
Also, shoutout to Kevin for being the best Nami.