what materials do i need to make bottle cap magnets?

You might want to try an epoxy or cement like G-S Hypo Cement. It is designed for gluing non-porous surfaces (like glass and metals). G-S Hypo Cement also does not glue your fingers together, which is nice if you accidentally make a mess. :)

If you want the name on the face of the cap to be the top of your magnet, make sure you get a magnet that is thick enough to reach beyond the lip of the cap when you glue it to the inside.

If you're using the other side of the bottle cap, and it doesn't matter what's printed on the cap, you may want to look into buying blank caps. They are available at several shops online like this one: http://www.bottlecapco.com/ordernow.html?gclid=CK7...

You need a bottle cap and a magnet. You glue the bottle cap to the magnet. Liquid Nails or Gorilla Glue would work well for this. Soda pop still comes in bottles with old-fashioned caps, but sometimes it's hard to find. Look in specialty food stores. The same is true of bottled water. If you're interested in beer bottle caps, they are extremely abundant.

Use a hot glue gun to glue them on. Or buy one of those super strong money glues. Hope this helps! :)