No man you got it a little off. Line up the kit and drill holes for your rivets. Take it back off, this will give you something to line it back up with. Then I would say use something like liquid nails on the surfaces that will touch. Kinda like glueing it to the metal. Put it back on and pop in your pop rivets. This will now have the two joined so they won't move. Then drill holes in the bumper and the peice you are molding to it along the seam they make.

Would look something like this:

O O O O O O O O O
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O O O O O O O O O

Now when you put the resin on it will seep into these holes and have something to grab on to instead of just making it look smooth. Then sand, sand, sand and sand some more. When you think you are done sanding, hit it with a little primer to show the low spots. Add some filler and sand, sand, sand, and sand some more.