1 gallon water 2 1/2 tablespoons pure-castile liquid soap 1 tablespoon vegetable oil (optional)
Fill a gallon-size spray container with water, then add the soap and oil. Mix or shake the container thoroughly before using.
The oil helps the solution stick around longer after being sprayed. Since the oil can go rancid, I mix up a fresh batch of insecticidal soap every time I need it.
If you want to keep some on hand at all times, omit the oil.
To scale the recipe for smaller applications, use 2 teaspoons pure-castile liquid soap for every 1 quart water.