Light Brown Sugar:
Combine 1 cup sugar with 1 1/2 tablespoon molasses
Dark Brown Sugar:
Combine 1 cup sugar with 1/4 cup molasses
*The exact type of granulated sugar is not important. It can be white, raw, turbinado, etc. as long as it is granules and suitable for cooking with it will work. Just keep in mind that different types of granulated sugar have different flavors which may affect the taste of the recipe you use it in.
**As with the sugar, there are different varieties of molasses which also have varying flavors. Blackstrap and Dark or Medium Molasses will have a stronger flavor. Light Molasses or Treacle will have a lighter sweeter flavor.