It is also possible to just cut out the decal/backing paper and use them as if you had put them on a piece of paper. Usually you need to use a magic marker that is belt color to "paint" the edges, since this will show white or the backing paper color, which is seldom the color of the belts themselves. Usually you do not need to color the back, because even if they are above the seat surface aways, they are still close enough that you cannot see the back from outside the cockpit.
For race car belts, I have found good images in google ads for various brands of belts, and have just resized the image, printed them on paper, cut them out and applied to seat.