The body is pretty straight forward, so I won't go into too much detail, but here's how I did the head and Mawloc spines. I used 3/16" magnets for the weight bearing joins, and 1/8" for the non-weight bearing.
Note that this is not the best way to do this. I glued two magnets into the bottom part of the head. You can see some drill scars from previous attempts. I then glued a paperclip bit to the front and back of the top part of the head. I carefully bent and cut the paperclip until it touched the magnets. The best way to do this is like the way I did on the Night Spinner, making a greenstuff setting and fitting the magnets that way. My way works, but it could have been better. I magnetized the jaw in this way, and the mandibles were really easy.
The body is pretty straightforward. Be sure to check your polarities. Each side needs to have the same polarity. The left and right can have different polarities or the same, it's up to you. I make them different.
Here's how I magnetized the Mawloc spines. I glued some 1/8" magnets into the inside of the body, and sealed them with epoxy. You don't want these to come undone and rattle around the inside. The magnets are powerful enough they'll hold the spines on even through the plastic. While the body is apart, drill and glue the arm sockets. You can reinforce the insides with epoxy in case you drilled too greedily and too deep.