I apply flames, shoot a coat of clear over them, then scuff it up with 1000 grit. Then I tape off any area of flame that I don't want shadowed by laying down some 1/8 fineline along the edges and graduate to 2" masking tape. Then I start airbrushing. It's kind of complicated if you have overlapping flames. FWIW, I over-reduce the paint for the shadows so I don't lay down too much at once. It let's me build up the shadow gradually.
With this procedure, if/when I make an error I can wipe off the shadow with reducer and not ruin my flames. Rarely am I satisfied with my shadows the first time (I'm not picky...just not very good yet,) so this method is a must for me.