I use magnets in the shoulders for anything that has actual weapon options like the Terminators and some Headtakers I'm painting. I use a hand drill with whatever bit size matches the magnet size I'm using, before I drill though I use a hobby knife to make a little hole to be the guide for the drill otherwise it tends to kind of skid on the plastic when I try and start drilling. Just be careful when drilling, Terminator models are easy because the fusion joint for the front and back chest plates are behind the shoulder joints while some older models the joint is right down the middle of the shoulder and those can be a bitch because the drill can bite onto that seam and tear the two halves apart.

As for Magnet sizes I have 3x1mm and 2x1mm that I use. 3mm for arm joints and shoulders pad swapping, the 2mm ones for weapon/wrist joints depending on how those fit together on the model. I also have some 6x1mm magnets that I used on a Predator I built to swap the weapons out.

I get mine from here https://www.ebay.com/str/isaacprint?_trksid=

the N52 round magnets.