For flames, spray the lighter color first, then do your masking and spray the darker color. Here is my recent flame job:
Search for articles online on how to paint flames. I found a few good articles when researching for mine. I didn't do it the ideal way on mine, but I will be doing a new tank eventually so at least I have some practice.
You can get a pretty good finish with just a rattle can. Some things to ensure this:
-degrease and wash the tank thoroughly FIRST.
-Sand the stock paint off the tank. I used 80, 200, 320, and 600. I didn't worry about doing the bottom well since you won't see it, I just scuffed it and cleaned it up with the 320.
-Use a good primer made for metal. (I used Rustoleum for everything) A couple coats, wetsanding between each is what I did.
-Use clean hands and a rag when handling the tank, or wear gloves. Only handle the tank by the bolts is the best way to not get gunk on the surface to be painted.
-if using just a plain color, wetsand between coats with min 1000 grit paper. At the very least, wetsand the 2nd-to-last coat.
-If you're using a metallic paint, don't wet sand the color.
-Don't lay the paint on too thick. My first coat or two are usually pretty thin/dry, and I spray them a little wetter with each coat, but not too wet - drips and runs are bad, mmkay?
-Lots of a good clear - not too heavy on each coat, not too thin. This takes a little practice. I used Rustoleum Lacquer.
-You can wetsand and polish the heck out of the clear if you want. With the lacquer and the way I applied it, I have a very smooth and glossy finish without doing anything after spraying.
Hope that helps.