If you've been handed a bag full of raw fleece from a shorn sheep, first you want to skirt the fleece.
If you're lucky, the fleece is still sheep-shaped (but seemingly much larger than the sheep it came from) and you can unroll it back into that shape. I say lucky, because skirting is alot like removing the outermost "skirt" of fiber from the fleece -- the outer 6 inches or so, usually. So if it can be unrolled, you can easily find the bits that need removing. Otherwise, lay it out piece by piece and examine them as you go...
What you are looking to remove (and throw out -- it makes a nice mulch, the birds like it, or simply fragrantize your trash can with it ...) is the extra dirty parts of the fleece -- the stuff near the tail of the sheep may have "tags" hanging from it. Don't ask what they are, just put them in the skirtings pile. There may be sections of fleece along the outer edges, and occasionally in the center back (so technically you'd be donutting the fleece?) that are so heavily embedded with hay, grain or weeds that you don't want to save them at all.
So, take all the skirtings, throw them out, and keep the best parts of the fleece to wash, card, and spin.
I have to admit I kept way more of the fleece in the first 10 fleeces I skirted than I keep now that I've skirted over 100 of them. You learn the time/fiber tradeoff from experience, choosing when it's worth keeping fleece or not. I toss anything that has: tips so muddy I can't easily open them in their dirty state (they won't wash clean the first time -- I'd have to clean, then pick, then re-wash the fleece to get them open) and also sections that have alot of visible hay/seed/weed/grass bits stuck in them, that would lead to bits in the finished carded wool.
Also, while you are skirting, you should get a good look at both sides of the fleece, since you should flip it to check it from both sides. Check for "second cuts" -- 1/2 inch or shorter lengths of cut fleece caused by the second pass of the shearer (thus the name second cuts) getting 1/2 inch or so closer to the skin -- these should also be removed, as they card into neps (little bumps) in the carded wool.
Also while skirting, you can test the locks in random places for soundness -- take a lock, hold it at both ends, snap it sharply together and apart to try to break it. If it breaks -- the fleece is weak, and may not stand up to carding, depending on how delicate you are in the process. If you card a fleece with breaks in it, your final product is likely to have neps or noils in it (lumps and bumps of fiber, not smoothly carded wool). The break may be local, in which case you can skirt it away, or the whole fleece may be weak; so test around an area that breaks, and test random locations in the fleece.
Whew. I can go on and on and on about skirting fleeces ...
There's a cool fleece diagram and more information on skirting fleeces here.
(posted by me on knittyspins, 23Mar07)
It must be that time of year again ... since a similar topic just came up too:
How do you get hay and twigs out of raw fleece?. I'll dig up my fleece washing notes for a posting tomorrow, so stay tuned for the next chapter on "raw fleece 101".