Basically you simply need to create the stencil shape on a layer, however is easiest for you.
Once you have the stencil layer it's a repeated process....
- place location of stencil
- Command/Ctrl-click the stencil layer thumbnail to load selection
- Choose Select > Inverse (Command/Ctrl+Shift+I)
- Deselect (Command/Ctrl+d)
It can be made a great deal easier with Photoshop though.
All you need to do is paint this once:
Then simply duplicate, move and transform the layer to create additional instances of it.
In fact, you could create a custom brush with that single figure and then simply brush on the various base objects.
However, individually painting each figure does offer some variation in the paint which will not be present when duplicating layers or using a custom brush. Using the stencil layer and repeating is the best method for some natural variation to the flames.