A wool soaker is not a diaper. It's a cover. You do need to be using a diaper still - if that's a prefold or a fitted. You cannot trifold a pf in a soaker - you need to snappi/pin it on, and you want to do your best to make sure it's not going to leak poo - because wool covers are hand-wash only, and they take a day or two to dry.
As for washing - there are lots of great tutorials around here - you also need to worry about lanolizing it at some point down the road. Basically what I do for washing is fill the wash tub with an inch or two of warm water, and a drizzle of wool wash soap (Eucalan or Kookabura - check your local yarn store, you only need the little bottle). Drop the wool in there so it's laying flat, dunk it under the water, and walk away. Come back later, lift it out, dump the water. Refill the tub with warm water (don't run the water onto the cover, move it out of the way), put the cover back in, swish it about a bit (not vigorously, and not a lot, you just want a little bit of movement through the fibers), lay it flat again and walk away. If it's been feeling crunchy or really stinky, I might repeat that a few more times (drain, rinse, swish, soak). If there's poo on the cover, after the first soak, I'll rub it gently to get it loose, and after hte second soak I might pull out my wool wash bar, give it a light rub across the spot and then try to work the poo out gently. Rinse at least one more time after that. When it's done, lift it out, squeeze it gently (don't wring it), and take a big bath towel, fold it in half longways, lay your wool on one end and roll it up inside the towel so that the excess towel is around the outside. Put the roll on the floor and walk back and forth on it to squeeze all the water into the towel. Now unwrap your wool, the towel should be thoroughly wet, and your wool should feel almost dry. Lay or drape it in a warm spot where it can finish drying.
The above does not cover lanolizing, which I suck at. Hopefully someone will be along with instructions for that.