I traced the bottom of the bottle cap (the smallest side) on my chosen paper, then cut it out.
Easy (and quick) personalized gift...and it cost pennies to make :) (especially if you have all the items on hand). You could also use other items instead of sea shells: maybe legos, marbles, old jewelry, etc. Really, anything small enough to fit into a bottle cap will work! And scrapbook paper, construction paper, postcards, fabric or pictures (like here) can be used in the bottom of the bottle caps.
Have fun personalizing bottle cap magnets for your next Birthday gift or how about magnets for a bridal shower made from the bride's choice of colored paper...the possibilities are nearly endless --->go forth and conquer the bottle cap magnets!!!