Grab your soap and squirt some into a cup. Fill the cup by letting the water hit the edge first.
Soak the brush by keeping the ferrule (metal part) out of the water. Swish the brush back and forth for a minute or two. See how dirty the water is?
Rinse the brush until the water runs clear.
Lay the brush on it's side on the towel and leave it to dry.
If you frequently get acne you should wash your brushes around 6-7 times a week, if this seems like too much of a hassle, keep some spare brushes so you can rotate through them.