Step 1

Take the split ring and tie one end of each elastic band to it so they are opposite each other, using the “loop inside” method.

Step 2

Open the card, and fix the loose end of one of the elastic bands to the inside back of the card by trapping it under a couple of staples.

Step 3

Pull the other elastic band’s loose end towards the bottom of the card until both bands are taut, but not stretched.

Step 4

Secure the second elastic band in the same manner as the first, but staple it to the bottom of the card.

Step 5

"Wind up” the split ring attached to the elastic bands, being careful not to over wind and tear the elastic out from under the staples, and then “trap” the split ring so it lies flat to the card when you close it.

Step 6

Tack down the front of the card to the back, as it has a tendency to try and pop open when the ring is wound up. Use a small amount of paper glue, or Blu-Tack to hold the corners lightly together, so that the card can still be opened.

Step 7

Pass the card to the recipient. Hopefully, he or she will open it, the split ring will be freed and spin around releasing its stored energy, and it will frighten the life out of your intended victim.

Tips

You can mount the rattler horizontally or vertically, as it won't really matter. Given most cards are taller than they are wide, mounting vertically usually makes the most sense.
The larger the diameter ring, the further the card will need to be opened before it “goes off”.
A split ring/washer with a rounded edge seems to work best, as it "skips" more easily across the card as it spins.
Don’t forget to use a plastic ring and adhesive tape in place of the staples and a metal ring if you intent to post the card in the mail. There's always a chance it'll be X-Rayed and pulled out etc.
Other things you can use could be a plastic tap washer, plastic ring that gets left on the neck of a bottle when you break a screw cap seal, or an odd piece of chain link.
Scout around car repair shops/forecourts for free washers/rings etc.

Warnings

Don't over wind the rattler, otherwise you risk the elastic bands pulling out from the staples under the strain.
Don't store the card for long periods with the rattler wound up, for the same reason as the above.
Always use fresh elastic bands.