Here's a tip for working decals on marine shoulder pads.
You'll want to use Micro sol and Micro set. You can get them at hobby towns, and some other hobby stores I imagine. Micro Sol helps soften the decal so it smooths out on the surface, then Micro Set, sets in in place, obviously. Works pretty well. They also help eliminate bubbles, apply them with a paint brush.
However it is still a very curved surface, I recommend using a sharp Xacto blade to cut lines in the decals. For ultra marines I'd cut it between the upper parts of the U, that should help it wrap around the shoulderpad quite well.