It was quite hard to find a detailed description of how to magnetize a land speeder. It is actually a tricky kit to assemble with magnets. So here is my contribution to the Internets. Hold on to your neckbeards, this will be a lot of steps and pictures.
Firstly, assemble the two main pieces. Don't glue on the sides yet, we will need those loose to start with.
These are the magnets I'll be using. 3 mm for the doors and 2 mm for the guns. I would have preferred larger, flatter magnets for the doors, but hey, this is what I had at home. :)
Cut out another piece of sprue and use the scalpel to get a near perfect fit between the inner walls. Dry fit until it feels good. Use plenty of thin plastic glue on the sides - this will even out the plastic and make the beam "float around" on the inner walls for a while, which gives us time to get a perfect fit.
Et voilà! The magnet came off without a hitch. Good work, me.
I hope you enjoyed this tutorial, or at least learnt something of value from it. Have fun out there, and remember, don't eat the magnets.