Most important Tips for Early Game:

Maintain proper wave management by freezing, pushing, or slow pushing the minion wave depending on the situation.
Freeze the wave near your turret to deny farm and set up gank opportunities for your jungler, but be mindful of where the enemy team is as smolder is constantly under threat of being punished early.
Push the wave when you have an advantage or need to reset the lane.

How to play the Mid Game:

Make informed rotations based on enemy movements and vision control to capitalize off of the enemy teams mistakes. As Smolder, you're much weaker until 225 stacks, so make sure to focus on poking the enemy with W, and farming with Q as much as you can. Set up waves to maximize your stacks.
Apply pressure on side lanes through split pushing while maintaining awareness of the enemy team's movements, hold your E and make sure to stay close to large walls so you can E over gaps that the enemy cant cross.

How to succeed in the Late Game:

After getting 225 stacks and around 4-5 items, look to stay grouped with team as you're going to be relatively squishy and easy to pick off. You are the most important part of your team in the late game.
Try to position behind anyone you can in order to dodge getting hit by CC, and play the fights accordingly.
Continuously Q the waves that the enemy team is walking in to apply your % MAX HP (true damage) burn, as well as constantly using your Q in team fights as well, weaving in other spells when you can.