Okay first of all, if you have access to a router, I would use that instead of a dremel, simply because it will be much easier to control. If you insist on using a dremel, at least make sure you use a router attachment for it.
Okay for the bit, you need a 1/16" slot cutting bit. I just did this procedure (but with a router) a few weeks ago. It's super easy. Took 2 minutes.
Yes, they are expensive. My buddy happened to have the exact bit I needed so I didn't have to purchase. But, it will run you probably $20-30 unfortunately.
Here is a website with a picture of what a slot cutting bit looks like:
http://www.woodsmithstore.com/slot.htmlThey are a little cheaper to order there, but they are not the right size. Get a 1/16" one.
Hope this helps.
Jakobud