Using soap or oil in a homemade insecticide is called a mechanical kill. It smothers aphids, unlike chemicals, which poison them.
Mix 3 tbsp. of dish soap with a gallon of water for small numbers of aphids. Alternatively, you can mix 1 tsp. vegetable oil, 1 tsp. dish soap and 1 cup of water for infested roses. Put in a sprayer and coat all plant surfaces twice a day until aphids are gone.
The addition of oil can make a spray coat a plant better, but it is important not to apply it during the heat of the day. To avoid leaf burn, only apply in early morning or evening.