the basics are to -
1, find a supplier of rare earth magnets as they are the strongest,
2, find a drill bit which the same size (eg for my
fw dreads i found that the lug hole in the arm is about 4mm so bought 4mm magnets and hand-drilled the body to fit in a magnet here too),
3, get your polarity right! a magnet has a north pole and a south pole - north attracts to south & same poles repel - i suggest you use a marker pen to mark the end you want showing,
4, glue in the magnet with superglue - maybe add some green stuff the make sure it fits 100%,
5, wait for everything to dry before seeing if you've done it right!
your assumption re the weapon options is correct. however for some awkard sized models (eg some cavalry) it can also be useful to make packing away the model between games safer & easier.
here's an eg of one of my dreads: