From the Warp: Magnetising the Space Marine STC
Pit of the Oni: Magnetising a Razorback
There are a couple of really useful tricks that I've worked out for myself while magnetising my tanks.
Marking Magnet Polarities with a File
A lot of the tutorials that I see suggest using spots of red paint to mark magnet polarities, and this works absolutely fine as long as you are patient enough to wait for the paint to dry. I've found myself using a file to mark polarities instead, which works immediately, and won't interfere with any glue joins either. Here's how to do it.
1. Take the piece that you've already fixed a magnet to, and stick an extra magnet to it. This magnet will eventually be glued to your new piece. In this case I'm working on a rhino door.
2. Take a file, and use it roughen the exposed side of the magnet. Often, the magnet will come off the model and end up sticking to the file instead. This is fine, because it should still be facing the right way, and you can then rub into the file by hand.
Be Consistent with Magnet Polarities and Positions
There's absolutely no reason why you shouldn't make each model you work on completely bespoke and put the magnets in different positions every time, but your life will be a lot easier if you are consistent.
For example, when working on tanks, I make all the magnets on the body one polarity, and all the removable parts the other. This means you don't need to keep checking polarities - each component can be "calibrated" against any magnet already mounted on the main body of the tank. If you extend this to your entire army, and are also consistent with magnet positions, it means that parts will be interchangeable between each tank and don't have to spend time looking in your case for exactly the right storm bolter or rhino hatch. In theory they should all fit.
Here I'm using a door from a completed rhino to mark magnet positions on a new one. The polarities will also be set using the completed door, making parts interchangeable between the two tanks.
This one's dead simple. Add a little collar of green stuff on top of a magnet that's superglued in for extra durability.
Another simple one. I like to put all the spare bits and pieces inside a tank when they're not in use. Just open up the magnetised rhino hatch and shove them in. One thing that won't fit is a Razorback turret when its not in use, but this is a handy way to make sure the little bits like storm bolters and hatch covers don't get lost.