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The easiest way to make flames is to first test your skill by drawing a bunch on paper. Draw little rockets with flames on them so you develop good flame shape, one that you will want on your rocket. Once you have done this paint your rocket a base color, then let is dry for a couple days. Go down to the body shop and buy some 3M plastic tape. The guy will get you the right stuff if you tell him what you are doing. This stuff makes curves like a dream. Put some on your rocket, it might take a couple tries, because it took me more than that. Finally I ended up with a shape I liked, then I masked the rest of the rocket, good masking is good because it really sucks when you get little spots of a different color.
Then spray on at least 2 coats and let it dry. if you want, when the paint is masked, lightly sand the base coat for better adhesion of the top coat. Once you have a good color let it dry to the touch. I then pulled off the rest of the masking tape because in the past, although rare, I have had regular masking peel paint off with it. After this you should just have your plastic tape, I would give this another day and then pulll off (if the paint isnt dry it might come up with the tape)
Now hit the rocket with clear coat, then wetsand the bump from the flames, do this over and over until the rocket is smooth. This last step is not nec. but if you feel the rocket you will feel the bumps, up to you, I didnt do it.
HTH