Well I found 2 recipes.. hopefully one will work for you. :0)

Bubble Bath (moisturizing)
1/2C soap flakes
1C boiling water
3T sweet almond oil
few drops essential oils (optional)
Dissolve soap in boiling water. Immediately add almond oil to water mixture, then EO. Stir constantly while you add 4-5 T of the solution to your bath.
The rest can be used later, but must be remixed. Shelf life one month.

I would put this in a bottle so you can just shake it. I don't know how much I personally like this one.. because unless it is moving while you pour it in.. it sounds like it separates. Not fun!
Next one! Which is the one I really think you are looking for. :0)

Basic Bubble Bath
1 quart water
1 bar castile soap, shredded
2 oz. Glycerin
3 drops fragrance oil

Add castile soap to hot/warm water. Mix until you have a liquid solution. Add glycerin and mix. Add fragrance.

This one sounds way better, but I thought I would give you both so you can decide yourself. Have fun with your bubbles!
Stacy of Lone Tree Soaps

The reason the recipes don't tell you how much to use is that if you're using soap to make bath water foamy, the amount required depends on the amount of bath water times the "hardness" of the water. In many waters it could require more than an entire bar of soap to make a full tub foamy. This is very different from conventional bath foams, where the amount you need is practically independent of the water. For most people, a bubble bath based on actual soap soap is not practicable. Although some people used soap powder or flakes to make a bubble bath before (or even after) foamier alternatives were available, to get enough suds that way you have to make the water pretty soapy, which will tend to dry skin and/or make it itchy afterward.