It’s been a while since I owned this figure, but like ZE said, take off the hands and the armor on one side. Slip it onto one arm and then loosen the strap on the other side to get it through the arm.
If the strap came out of the buckle, I find it easiest to get a tiny bit of scotch tape and wrap it around the tip of the strap at a slight point. It’s easier to pull the tape through the buckle than the strap itself.