Depending on the size you can use a can lid or a canning jar lid to attach it to.....cut a circle and use a running stitch....add magnet....hope this helps.
Gayle, that sounds like a great magnet - why don't you just use a satin stitch around the outside - then use your pinking shears to cut around the edges and glue a magnet on the backside.
here in the netherlands you can buy a magnetic tape. One side is with glue so you can glue on backside, the other side is the magnet.
Sure, visit Office Depot, Staples and purchase the sheets of magnets, cut to fit the size you need and glue the back on using Fabri-Tac. Also look in your real phone books for the attorneys that use these magnets to hold their ad on; these are FREE