I just got my first fleeces last week!
I started with the shortest & dirtiest bits first-thinking if I messed up, I wouldn't have lost the good stuff. Before I even started, I took some and intentionally tried to felt it, so I'd know how much I could actually handle it before it turned into a huge mess.
I soaked the longer, better parts to loosen up the dirt & 'mud'.
My biggest mistakes were:
Not sorting well enough by length.
and Expecting ALL of the VM which was too small to pick out to completely card out. I was carding FOREVER until I read something about commercial fibers that have been treated with some sort of chemical which 'dissolves' the VM out.
I'd love to hear from anyone else who has done this!