Spread your canvas tarp outside. Mix dish-washing soap with warm water in a bucket.
Dunk a sponge or dishcloth into the soapy water. Remove the soaked cloth and then scrub the outside of the tarp.
Rinse the tarp with water and then hang it on a clothesline. If the clothesline is a double line and the tarp is very large, extend it across both lines. Scrub the other side of the tarp and then rinse. Allow the tarp to hang dry. Do not put the tarp into a dryer.
Place the tarp into a large sink or in a tub. Run water over it until the material is submerged.
Add soap to the water and mix the water around. Rub the tarp with a cloth or a sponge on both sides.
Drain the sink or tub and then squeeze the excess water from the tarp. Hang the tarp to dry. Do not put the tarp into a dryer.