This is relatively easy! If your royal is extremely dark colored, make sure to spray a white "ghost" pattern, very lightly, onto the cake. Then begin your overlays, starting with the red on the tips, then orange, then the yellows. Make some colors stronger in a few areas. If you have some flame stencils (the odd shaped stencils-not ones that look like flames), use those to give the flames some natural curl effects. If you choose to outline in black, you can do that last, then make any "tweeks" to the color saturation to make them realistc. The white ghost layer is very important on dark colors as the other colors will disappear if you don't use white first.
Hope this helps!