Make sure you've got the area you want it to work in marked on the shell. Just a box will do, then you can get it in the right place easy.
If it's a real detailed one I'd tend to transfer the entire piece to the shell then once in place weed it out as it will carry better unweeded.
Make sure you cut too many as well ( the beauty of having your own cutter ! ) and then when you mess the first 2 up you still have spares to use
I will say putting things on the inside backwards is a complete arse to do though, I hate doing it, applying to a backwards curve is just a ****.