If you'd like to have symmetrical flames, this is one way of doing it. There are other ways, but this is one of the easiest.
This example would be used on a car hood and two fenders.
Make a mark down the center of the right side and left side. Now place a piece of paper (I usually use 36" masking paper), on one of the half's. In other words so it covers the hood. Place another piece so if covers the fender. So now you have one side completely covered nice and neat in masking paper. Now mark a reference point at the peak of you fender.
Pull the paper off and lay it on a rubber mat. Draw your flames out. Now use a Dress-makers wheel (don't know the exact name of it, but it is a small wheel with spurs on it). Now you will need to take the tool and run it around your flame outline that you just drew. This tool will make perforations around the outline of the flame.
Now lay your paper back on the hood and fenders using the reference points. Use some chalk powder and go along your outline of flames (where the perforations are). When you remove the paper, you will see the outline of your flames on the car's hood and fender.
Now you will need to flip the piece of paper over and use the other side for the other half of your flames. Do the same--line up your reference points, then use the chalk.
Now you should have a complete outline on both fenders and the hood. Now you can run 1/8" tape just inside of the flame outline that is chalk.
This will give you symmetrical flames. This is pretty much the basics. This should give you an idea of how to do it.
There is another way of doing the above using "spray mask", but that is another story.
Good Luck--hope this helps!
Scott