First, gather up your supplies:
3 1/2 quarts water
3/4 cup Borax
3/4 cup Washing Soda
1 cup castile soap
20 drops essential oils (optional) (I like lemon, lavender, geranium)
Heat the water, borax, and washing soda on the stove (it does not need to boil) until completely dissolved. Allow to cool.
Pour into a container (I use a gallon jug) and add the castile soap and the essential oils. Shake to mix. I use about 1/4 - 1/2 cup per load of laundry.