Soap Calculator Spreadsheet
(Note: This spreadsheet appears to not work in newer versions of OpenOffice/LibreOffice. It actually does work. It is merely the navigation links that don't. You can get to the “Bar Soap” page by going Edit→Sheet→Select and selecting the Bar Soap sheet. In some versions, you can use ctrl-click to navigate!)
Or, you can use this online calculator: http://soapcalc.net/calc/SoapCalcWP.asp

Basic Recipe:
Oil
Water
Lye (Sodium Hydroxide)

Carefully weigh Oil.
Multiply by SAP to determine amount of lye. (Make sure to use NaOH values, not KOH.)
Use 2.7g of water per 1g of lye.
Discount lye for superfat.
Discount water as desired.

Slowly add lye to water. Do not allow it to get too hot! (Start with 1/2 ice)
* NEVER add water to lye *
Slowly add lye solution to oil, stirring constantly until trace.
Pour mixture into mold and allow to partially harden. (24h)
Cut into bars. Allow to cure for 4 to 6 weeks.

More involved order of ingredients:

Lye solution:
1. Water
2. Sugar (for clear soap)
3. Salt
4. Ammonia
5. Borax
6. Lye

Oils:
1. Melt any solid oils/fats.
2. Keep “superfats” seperate.

Start mixing.
1. Lanolin (Max 120F, otherwise it will curdle)
2. Coal Oil
3. Lemon Juice

Mix until trace.
1. Add ground oatmeal or spent coffee grounds
2. Vitamin E
3. Colouring dyes
4. Superfatting oils
5. Scenting oils