The interchangeable parts for the warjack are the weapon arms, shoulder pads, heads, and the armored shroud covering the head. The basic process for all the bits is the same - drill a hole slightly bigger than the magnet as deep as possible in each side, fill the hole with a tiny bit of superglue and Green Stuff, press the magnets into the hole, and remove the excess. You have to make sure the poles of the magnets are aligned properly.
Magnetizing the shoulder pad
The middle part of the arms of this model were really weak and kept flexing - I built them up with several layers of Citadel's Liquid Green Stuff, and will file them into shape when it cures.
Magnetizing the front carapace
Filing excess Green Stuff after it has cured
Drilled out the barrels of the cannons on the Spriggan shouldpads
Mostly finished. Since the head is almost completely concealed by the armored carapace, I opted to just use a single head, since magnetizing it would be awfully fidgety, and they're so small that they'd be waaay too easy to lose
...and that's about it for the build. Next I have to find a way to prime it without covering up the magnets...