put a small pool of hotglue in the bottom of a bottcap. this makes it easier to keep the magnet from jumping out of your grip when you go to stick it down... and my magnets weren't thick enough to keep the sharp edges of the bottcaps from setting directly on my fridge. if your magnets are thick enough you don't need much glue, but it definitely helps. if your magnets are thin put a nice layer of glue in the bottcap so the magnet stands off enough.
after the glue has dried, put a small dot of hotglue on the magnet.
while it's still hot, press it into the bottlecap and hold it so it doesn't dry slanted or anything.
I wasn't sure of the holding strength of the amount of glue I put on the magnet and how well it'd bond to the already cooled glue in the bottcap, so I ran more glue around the edge to seal it in a bit more after it was cool.
let that cool and stick 'em up :)