I found instructions on how to do this on Karen Clark's site Fun Activities for Kids. Here's what you do:

Step 1: Gather Intelligence. Visit Santa at your local mall or department store. Listen carefully to what your children tell him! Or stay home and help your kids write a letter to Santa.

Step 2. Sneakily compose your reply letter from Santa. Incorporate references to the kids' conversation with Santa or the contents of their "Dear Santa" letter. Handwrite or print it, and use whatever craft tricks or computer skills you like to make it look authentic.

As example of just how far do-it-yourself thinking can go, here's an inspiring story which I picked up from AutoblogGreen about an electric Volkswagen Super Beetle and the Alaskan teen who is bringing it to completion. Started as a response to a school project which challenged students to come up with creative solutions to our global climate challenges, Bart Grabman's electric Beetle has served the young fellow as a priceless tutor in the realms of product development, prototype and assembly process. In a story reported by By Erin Britton, McClatchy Newspapers, via TuscaloosaNews.com, Grabman stated, "It's been an interesting experience trying to figure it out ... I'm not done with it, so I'm still learning a lot."