A quick visual guide
Starting here with the circuit board for the helmet already extracted and the back of the calculator removed.
There is only one screw keeping the entire keypad assembly together. Remove it.
lift off the electronics board and you will see the rubber backing underneath
Now, pull off the battery leads from the board. You can easily do this with your hands.
Then remove the rubber backing
Then carefully remove the black plastic backing and you'll have all these components laid out:
The keypad keys are now being held in by gravity so do not turn over the calc body or move it. Keep the keys in place for the procedure.
Next remove the battery bay from the black plastic
then put it back on the keyboard
Then the rubber backing. Make sure it's tight against the keys and making good contact with all keys.
Place the electronics board back and secure with screw.
Then you are ready to cut the excess areas from the keypad. The below shot is a rough cut with a dremel.
After this, sand down what's left on the sides until the black striping is all but gone. I think the originals are quite thin on the edges, especially for the ROTJ hero, but current gauntlet replicas (as of mid 2023) are not exactly correct on the dimensions for the tray, so make sure to check your fit before going too far.
You can cut the three tabs or overhangs on the electronics board at any point. I cut them off at the end. Doesn't matter. At this point you're almost done. For extra credit, you will want to cut the bottom corners off the front and back ends.
very carefully cut at angle that matches the topside angle (45 degrees?)
Not only will it sit better in the tray this way, this is how it was done on the PP2 and the RotJ gauntlets, which probably means all versions were this way, including ESB.
Now you're done. Good job.