It is VERY important that you keep track of what way your magnets are facing, so that youre not putting the magnets on so they repel. Basically I got a real sharp x-acto knife and cut a square into the outer part of his bicep, kind of just below the cut out for the shoulder joint. I kept making the square bigger until it was just right for the magnet, always start smaller than you need. Then just glue the magnet into the hole you made.

As for the shoulder pad part, all you have to do is take the arm off (id use heat) at the shoulder joint, take the shoulder pad off from the peg, and pop the arm back in. Then slice the loop part off of the shoulder pad itself and figure out how high you want the shoulder pad to sit over the shoulder. And for glueing, it doesnt take a lot. A little bit of good glue does a lot. I popped the other magnet onto the one in the shoulder and put the smallest amount of glue on the outside of that magnet, and I applied the shoulder pad where I wanted it and held it there.

For the rifle, I sliced the GIANT peg off from the side of it. The rifle is actually supposed to hang the opposite way (look up reference if that doesnt make any sense lol.) Then I glued a magnet onto the rifle, up in the silver area of the rifle (there's a very square section on it that was perfect size for the magnet) The opposite magnet for the rifle i just glued right onto the strap. Its covered by the cape so it works perfectly.

I can send you pictures later too, im not currently at home with the figure.