Create a large, flat, clean space by hosing off a driveway or other non-dirt-based area.
Shake the tarp to remove any loose dirt and debris.
Spread the tarp out flat.
Use a push broom or scrub brush to brush off any dirt that is left on the tarp.
Mix two gallons of water with a few squirts of dish soap or other gentle soap in a bucket. Add one cup of vinegar for each gallon of water if you are trying to remove tough stains.
Use a stiff-bristled scrub brush or long-handled push broom dipped in the soap solution to scrub the tarp.
Rinse the soap off immediately with a garden hose, making sure no soapy residue is left.
Leave the tarp flat and let it dry completely before folding it.