Ever notice how 90% of the posts on a real question ar a bunch of jokers or usuelss crap? Well, here we will try to fix that:
Make a template of the flames, or better yet, check around in some auto magazines and catalogs like JC Whintey, etc. They may have premade templates.
Paint the item whatever bas color you want, let it dry, and then attach the template/mask. Spray on the color you want for flames, and get 100% coverage at the edge of the template. blend/feather the paint so it doesn't fully cover the deeper you go into the flame area until it is almost wispy.
After that, you can always shift the template down a couple fo inches and then do a wispy/hazy spray of a lighter color, i.e. start with orange for the flames, then use yellow for the second layer.
If you do it right is should look pretty good...