First off, once you receive your magnets you will need some cyanoacrylate super glue and a small piece of rubble. For flat frags such as montipora caps I try to find a piece of rubble that will sit on the magnet and have a surface that is somewhat flat and at 90 degrees to your work surface. It does not have to be exact but the flatter it is the easier it is to mount your monti cap. For any other frags, though, I try and find a piece of rubble with an interesting shape and only one flat side.

Separate your magnets and put the other magnet a fair distance away from the magnet that you plan on gluing (a foot or more away). These magnet are quite powerful, and if yours are close together and one gets nudged by accident once the glue is applied and your magnets stick together you may have a very sticky situation on your hands.

Apply liberal amounts of super glue to one magnet.

Place rock on magnet and press firmly.

Now at this point the longer you can allow the super glue to dry the better. Allowing the super glue to completely dry overnight is preferable but not required. Once your super glue has sufficiently dried you can pick up your piece. Now apply liberal amounts of super glue where you plan on attaching your frag.

This is where the technique of attaching your frags will differ. I chose a piece of cap as it is the most challenging to attach.
Dry the bottom of your cap with a piece of paper towel. I usually just put my cap on a piece of paper towel and let it sit there for a few minutes.  Turn your cap upside down and place your magnet + rubble + glue onto it.  Be sure to give yourself a bit of space between the frag and where the glass will be once its in the tank.

Allow the glue to set for a few min and place your piece in the tank.
Now this part is important. When you put your piece into the tank keep the magnets separate and do not allow the magnets to get to close to each other. I find it easiest to place the frag in the tank and figure out where i want it while the other magnet is in my pocket. Once you know where you want it for sure grab the other magnet and place it on the other side of the glass.

On one of the first magnets that I put together I made the mistake of allowing the magnet to get too close together while I was putting my frag into the tank. The magnets jumped together and pushed my frag off of the rock. Both my magnets and frag and subsequently fingers were covered in superglue so it would be great if I can help you avoid making my mistake.

If you are attaching a frag with a flat side already it is much easier and faster.

Here we took a piece of encrusting orange monti that we fragged off of a very large colony here. We cut the piece in half with a diamond wheel on a Dremel to give us a nice flat surface.

Again apply your super glue.

Place your coral on the magnet.

And you are done!

You can purchase Mag Mounts or Thumb Crushers Here.