I second what Pete says about the legalities. You'll want to double check those to make sure you don't get yourself in any trouble. Assuming you've got the legal issues worked out and a place to teach, here are some thoughts. I'm not much of a baker but I am a big bread geek and I often teach cooking classes at my restaurant.
Length of Class
A good starting point is 2.5-3 hours. You can pack a lot of info into that period of time (especially if you sweeten the deal with a recipe hand out and such), but after the three hour mark, it's easy for you to become weary, and the average "student" will be so satiated with information that they won't really retain anything after this point. I have done "boot camp" style classes that span a few days and last about 5 hours a day, but "students" are carefully screened to insure that they're the right fit for this style of class.
Cost of Class
It really depends on the situation and what your local market will bare. Starting out, give good deals to get the word out, but once you're established, here's a pretty standard breakdown: Most private cooking instructors will have a per/person, per/hour minimum. Ex, $50/hr per person with a two person minimum ($100/hr). That might sound like a lot, but remember, that usually will included any ingredients, use of tools, e-mail correspondence, printed recipes, etc. In fact, I think it's fairly low. You can also offer price breaks for groups say over 4. All day classes should have a flat fee with some bonuses like wine and dinner "thrown in" to sweeten the package.
Get The Word Out
Start a blog with free recipes, photos and videos if you can. Have an e-mail opt in newsletter and keep it up to date. A bi-monthly, e-mail newsletter is a grate way to keep people interested. If you have a baking business then that is best case scenario; people will taste your product and think to themselves "I wish I could bake like this."
If the above isn't possible, then you'll have to do it the old fashioned way; friends, family, giving freebie lessons in hopes of a referral, etc.
Good luck. Teaching can be an extremely rewarding and lucrative part of the culinary industry, although it is slightly difficult to get the ball rolling (like anything in this industry is easy!).
