http://www.dakkadakka.com/wiki/en/Jamunition%27s_guide_to_magnetising_your_models\
This article covers Terminators but works as a guide to Tau.
http://www.dakkadakka.com/wiki/en/Magnetizing_Terminators
A magnetized Tau army
http://www.dakkadakka.com/wiki/en/InquisitorMack%27s_Tau
I would add two pieces of advice.
A. Buy two or three sizes of magnet. Rods and thin discs are both useful.
B. You can make sure all the weapons are swappable to all the suits by careful selection of which magnet to stick into a hole.
Here's how to do this.
1. Stick a magnet into a suit.
2. Let another magnet attract to the first magnet.
3. Stick the second magnet in a weapon, without turning it over.
4. Use the magnetized weapon to pick up another magnet.
5. Stick the new magnet in a suit without turning it over.
6. And so on.