Gather all the cards you want to use - personally I keep them all and then use the ones I need when it comes to it...you never know what size or shape future presents will be!
All you need to do is take your cards and cut them in half, i.e. remove the side that has the message in, and discard it. Trim to size if need be - obviously small cards work the best but I do use large cards too.
Punch a hole in the top of your cards...
...and file them away until next Christmas!
When the time comes, add some ribbon / thread through the punch hole and attach to a present with your written message on the back.
Free gift tags ;)