Fill a washtub or bucket with warm water and laundry detergent. Allow the clothing to soak in the solution for an hour before attempting to wash.
Wash the clothing by hand in the detergent solution and wring out excess water. Concentrate on removing dried clumps of drywall by carefully pulling the material off the clothes.
Hang the clothing on a clothesline or fence. Spray stubborn areas where drywall compound remains with a spray nozzle attached to a water hose to loosen the material.
Place the clothing back into the detergent solution and continue washing by hand to remove any remaining joint compound or residue.
Wash the clothing in a washing machine on the hot water cycle to finish the process.