So, when I first posted my RnR and Clutch short sleeve mods, I've had a couple of requests to post how it's done. Here's the threads for the original customs of them:
I finally found suitable arms for Falcon and did it for him finally too. Here's some WIP pics and steps.
First, remove his hands and lower arm. Cut the cuff off the arm. Then make a hole down the middle.
Then on the upper arm/bicep, you have to trim around the bottom as shown:
This is so the cuff can fit over it.
The cuff is stretchy so you can stretch it over it (just trim more if it doesn't fit) and that holds it in place.
After you are happy with the fit of the sleeve, then you just pop the arm you want in there onto the figure.