The ones I've used required you to make 2 small slits in the fabric and put the 2 "legs" of the catch through, use the piece of plastic behind the fabric and use pliers to bend the 2 legs over the plastic backing.

I have used both types. The square ones are not so hidden as you have to sew a little envelope type of holder to slip them into. Depending on the fabric, use, and strength of the magnets you maybe could glue them, but magnets cause a lot of pull and the glue may not hold.

Sorry this reply is late, but just started sewing and checked out this forum. I'm in the middle of making a bag with this hidden magnet, I just sewed through the plastic right onto the material. Seemed to work fine so far!!