Items you may need are a dremel or pin vice and small magnets. The magnets I used are something like these
http://www.ebay.com/itm/20X-Super-Strong-Rare-Earth-Neodymium-Magnets-8mm-x-1mm-/170610874379?pt=LH_DefaultDomain_0&hash=item27b933580b
http://www.ebay.com/itm/100-Magnets-1-8-X-1-16-Warhammer-Tau-/150671114746?pt=LH_DefaultDomain_0&hash=item2314b2c1fa
You can Google search or e-bay search for the size you need. The prices are quite reasonable, the second link is the size of magnets I used, however I am not sure if that is the seller I bought from.
As for installing them, I used a dremel and drilled out a hole the same size as the magnets to where they would sit flat or slightly below the surface. A little putty on the edges and sanding to make the surface smooth, in some pictures you can still see a little circle where the magnets are but it is hard to tell if you are not looking. I think it is better to have the magnets set in a little to far and fixed with putty rather than sticking up a little.
Be careful to keep the polarity on the magnets all the same so you can change the load out without having weapons be repelled! I had to pull out a magnet and turn it around once and it was a pain!
You may want to use a sharpie and mark all the magnets you are going to use on the same side to avoid confusion.
Here is a suit I did (based on a suit seen at Adepticon) you can see a little circle from the magnet on his right arm, but it will usually have a weapon on it. I plan on repainting it one day....maybe when the Tau get updated.