Here is one site that tells you step by step, even has some pictures

http://www.dragonfarm.us/ConScul%201.html

Marcia
Good luck with your Birdbath and be sure to take lots of Pictures!

Sherri Warner Hunter has two great books on concrete garden ornaments, including birdbaths, with easy instructions and photos. Find them at B&N or Amazon or where ever. I just got back from a week-long class with her at John C. Campbell Folk School in Brasstown, NC. Awesome. The birdbaths we all made came out beautifully. Good luck!

I looked at the dragonfarm site with interest. Their system no doubt works, but I am not into making anything that fancy and involved.
Check out my response to "leaf birdbath pedestal" on 7/27/07. This has worked for me. I forgot to say the plastic cone is removed as soon as the mix sets up.
The pedestal could even be made completely solid and/or around a hardware cloth form for strength. You will need something like the black plastic cone to retain the shape until set up.
The bowl can be made upside down over a round form made from a 1 1/2 inch thick styrafoam board or large plastic saucer as a mold.
The outside of the bowl can be made perfectly round by making a 4 inch high "fence" from a large nursery pot. Just fill inside the fence, and then build up the center portion like a shallow truncated cone that is sized to mate with the top of the pedestal. If you cut a template from thin plywood shaped like you desire for the shape of the bottom outside of the bowl, it can be swept around the upside down bowl concentric with the center. This will make a uniform shape all around.
After turning over the bowl the inside surface can be coated with two coats of very thin mix of three parts sand to one part cement. This holds water fairly well.
Also the bowl could be made right side up over a form designed to mate with the pedestal. A large colorful glazed ceramic platter can be molded right in the mix. This will make a bowl that holds water perfectly and adds color.
The outside of the pedestal and bowl can be made as smooth as desired or it can be roughed up and gouged for a rustic look.
Actually any system that works for you is just fine.

I'm working on bird baths as well--I've been irked with the fact that bought ones don't seem to be planned with birds in mind! THey are too deep and too steep. What I did to get a nice slope was pack wet sand in a large terra cotta plant saucer, lay cardboard over it and invert. Then I used a loop of wire and 'shaved' away the corners until I got a slope I liked. Cover with plastic and add 'crete. My edges were ucky since I didn't have enough sense to make a mold or even draw a guideline for width. Good thing 'crete is cheap!