This is best done using a hole punch, but my office doesn't have a hole punch. It does, however, have a drill press, so I used a 5/16" drill bit. Use whatever you've got on hand - a Dremel or an exacto knife would be fine as well.
Make sure to punch (or drill) your holes about one hole diameter away from the card's edge for maximum strength. Sand off any rough bits left from your hole punching.
Note: if you've got lots of hair, punch the holes in opposite corners of the card - the resulting diagonal will hold more hair.