Fairly easy. I use some BIG magnets, and they're strong enoguh that I fight them every time I swap out the arms. Instead of using the 'cuffs' at the arms that let them swivel, that's where I put my magnets. It's a little tricky for the Battle Cannon and the Melta, because they use the same general gun casing, with different shield/barrel, so you need to magnetize the shield to the front of the gun, and settle on having a melta with a shell vent, or a battle cannon with fuel tanks.
Otherwise, every weapon arm is magnetizable. You have to pick one of the two missile options for the carapace.