I've found this tutorial for a model car. The same masking can be done with tape. Just keep in mind that since you're painting the inside of a clear shell, the layers of paint you put down first are the ones that will show. So you'll basically have to do everything in reverse. You may also be interested in this site for more detailed flames. You could also just put some flame decals on the inside if you don't want to trouble with masking and painting.