For my poster I want to add flames to the building which will be created in post-production. To create this I first selected the paint bucket and filled the background black. Afterwards I created a new layer that I, next, drew a wavy path on using the pen tool on the path selection. If necessary, I would use the path selection tool to move the shape around. 

Making sure that Layer 1 is selected, I'll proceed to Filter - Render - Flame. A panel will appear which I'll use to create my flame, depending on how I want it, with various options. 

Optional depending on the type of flame, if I wanted to create a "glow" for the flame, I'd create a new layer, and select the brush tool. I'd bring the hardness down and the size will depend on the size of the flame and/or how big I want the glow.