Ever since I got my little British bike I've been wanting to put a Union Jack decal on the tank. My first attempt failed miserably because I didn't account for the curvature of the tank at all. My second attempt also failed miserably because I tried to do my measuring with the decal (not affixed) on the tank. This is the third attempt and I took a different tack. I first created a template of the parts of the tank that would allow the decal to lay flat. Here is how it went...
I decided to use wax paper for the template. This was mostly due to the opacity of wax paper allowing me to see the tank while tracing the template. Also the wax paper would lay on the surface of the tank in much the same way as the decal. This way I could see the spots where the decal would get creases and avoid them. It was also very easy to cut the hole for the fuel cap.
I laid the wax paper over the tank and taped it down with some masking tape. First I marked out the hole for the fuel cap.
Next I placed on hand on the center of the tank and slid the other hand down the wax paper until I went passed my desired decal size or the wax paper started to bunch up.
Marking these spots allowed me a general idea of the limits for my decal. This allowed me to draw a smooth shape for the outline of the decal.