Make sure the scratchplate is totally free from grease, wash in luke warm water, with a little washing up liquid, rinse well leave to dry. Put your sheet over the plate, working from the centre, start to press outward, so as to not get any air trapped, then to trim, cut it around the outside as close as you can, then you could use a small file to cut around the bevel, using careful downstrokes, like removing pastry from a pie lid. Tart up (no pun intended) the edge with some fine sand paper using the same method.
I'd experiment on something similar and take your time, looking forward to seeing the finished article.