I've done it two different ways before, but both ended with at least a couple of coats of clear over the decals.
The first way was to put the decals on the base color coat and then clear over them. The second way was to put the decals on the first coat of clear and then a couple more coats of clear over them. Both ways worked out well. You need the clear over the decals to protect them from gas spills and the elements.