Set the nozzle to a spray setting and hose off the garage door as soon as possible after the Silly String meets the garage door surface. Try to hose off the Silly String before it dries for best results.
Fill the bucket with warm water and add two or three squirts of dish detergent. Swirl the water around until you get a bucket full of bubbles.
Dip the scrub brush into the soapy water and scrub at any areas of the garage door where Silly String hardened. Work the brush back and forth to loosen the Silly String. The soap may dissolve the Silly String or it may simply loosen it from the garage door surface, depending on how thick the layer of Silly String on the door is.
Spray the garage door again with the hose after you scrub off all of the Silly String.
Assess the garage door after it dries. Depending on how long the Silly String remained on the garage door, you may need to repaint areas where you see discoloration.