Why should you play Smolder?
Smolder is a scaling beast, that in late destroys everyone, but you need to play very safe at the early phase of the game. He is very flexible, you can pick him with engage supports, or ardents.
Runes
Fleet Footwork Is the best rune to choose, because it gives an advantage on early trades, or allows you to take farm more easily.
You Need a ghost to disengage more easily because you need to be very safe as Smolder.
Exhaust is pretty good too.
Quick tips
Remember to Ban in patch 14.2 Miss Fortune, which is very strong and stomps Smolder. Look what your support will pick, and play around him because when you stay solo, you will get dived every time. You want to avoid fights and unnecessary damage but keep poking. Focus on the farm until you get Sheen, later Essence Reaver, and some stacks.