Washing fleece can be achieved in a number of ways. By soaking in cold water to retain the grease but remove the dirt. How long the fleece is soaked depends on how dirty it is. Soaking the fleece in hot soapy water, to remove grease and dirt. Again, it depends on how dirty the fleece is as to how long it soaks. I usually leave the fleece overnight and rinse next day.
Fleece can also be washed in a washing machine, though it is again only soaked and not agitated or you will end up with a felted mess. Using a washing machine can take the weight out of lift the fleece if you are washing the entire fleece, or a large amount. The water can be drained, then refilled, squeezed by hand to remove the residue dirt, then spun for a short while to remove most of the water.
I also use a bath to wash a whole, or a few kilos, of fleece, leaving it to soak overnight and rinsing next day. Again, I spin out the excess water in the washing machine. The fleece dries so much quicker.
Below is a video of fleece washed in a washing machine. Sometimes, if fleece retains wax it will need more than one wash, and normally in hot water with as little agitation as possible to avoid matting. This video shows the second wash to remove the excess wax.