You should already have a sketch made with the flame pattern you want.
Copy the pattern onto the tape with a pencil (I’ve used pen because it photographs better, but traced my pencil marks). You can erase any mistakes with a good white eraser, so don’t be afraid to do so. Just make sure you don't lift any of your tape overlaps.
Pull the tape off, and place it on a cutting board. Note that if you’ve used multiple layers of tape as I have, you must cut “down” the tape, so you don’t lift your overlaps.
Carefully cut out your mask. This is the most important step in getting nice flames, so don’t rush it!!
Position your mask on your piece, and burnish the edges well.
Then paint, using multiple light coats…I used 3 or 4 to get the colour I wanted. As soon as you’re satisfied with your paint job, pull your mask off. I wait a few hours, then ¬gently go over the flames with 5000+ grit sandpaper to minimize the paint lines at the mask edge.
That’s it! You may need to do slight touch-up on your base coat.
You could so this several times, to create multi-colour flames, or flames with shadows (using this method means the shadow layer is the first one to paint) but you’ll have to make sure you don’t paint too heavy, or you’ll get raised paint lines, and you’ll need to use high-grit sand paper between each coat (if you do that, try to avoid metallics—they don’t sand so well).
