Here is a recipe for my favorite laundry powder. It is written using the simplers method. Just choose a quantity- a cup, a teaspoon, a half a cup. When it says "one part" use one of that quantity.
1 part Borax
1 part baking soda
1 part grated castile soap
10 drops essential oil of choice
Combine ingredients in a bowl and transfer to a paper bag or glass jar. Store away from heat and light. Use 1/2 c. for every medium sized load. Add or subtract amounts depending on how much you are washing.