Washing wool and other animal fibers can be tricky. If you do it wrong you get a glopping wet mass of felt. No fun at all and potentially costly if not a waste of time and effort. Some fibers are easier than others to wash and are more tolerant of agitation but all can and will mat up if mishandled.
This is a dead simple way to accomplish the task. There are of course other ways to wash fiber and wool can even be spun from the "grease" with no washing at all... but that's pretty nasty. Some animals are cleaner than others and some also tend to just be naturally kind of clean overall but they all smell and even if your unwashed fleece is tolerable when received, it's going to smell when it hits the water.
That said, other than being stinky it's a pretty easy job and requires almost no "tools". As mentioned above, there are several ways to do this. There is even a great Instructable that shows how to wash "dog fur" in a far more modern way that is identical to methods for washing wool.
I learned to process wool in a living history setting for demonstration purposes and have done it countless times. We couldn't use mesh bags or washing machines. Definitely labor intensive but if you only process and spin just a little at a time it's fine. So, that's what I'm showing here.
An important note may be that if you ask around long enough you might be able to get fleece or other fibers "for the asking" but it will almost certainly not be washed. If you are spinning or knitting on a budget this comes in handy. There is also a certain degree of satisfaction in taking wool "from sheep to shawl".