By way of introduction ...
Combs pre-date hand cards as a fiber preparation technique by thousands of years. Combed fibers produce 'top', where the fibers are all aligned parallel to the length of the fiber 'sausage' (aka top). When top is spun, very little air is trapped in the fiber, producing a denser yarn often referred to as 'worsted spun'.
When you comb, the shorter lengths of fiber will be left behind on the combs; only the longer, uniform fiber lengths will be in your resulting top. This is one of the intents of combing, to remove shorter fibers, matts, and anything else that is not suitable for the resulting top. The left-behind fiber can be set aside to use for carding, felting, or mulch --whichever seems most appropriate.
Fiber suitable for carding usually has a staple (per-strand) length of 3 inches or more. Wool, mohair, angora, cashmere -- pretty much any fiber with enough staple length is combable.
Start with washed fleece. It should not be matted, and relatively VM-free (VM is vegetable matter, those bits of hay and burrs that our fiber friends seem to love getting in their coats). The staple length (length of individual fibers) should be 3 inches or more, and should be relatively uniform throughout the fiber you are combing.
(Note: these instructions are written by a right-handed person, if you are left handed, you may find switching directions more comfortable.)
Hold one comb in your left hand, resting it on either (or both) leg with the tines at the edge of your leg. Take a lock with the shorn end pointing toward the handle, holding it parallel to the handle (i.e. horizontally) and put about 1/4" of the shorn end "behind" the tines, pushing the lock down the tines to the base of the comb. Repeat this, placing locks along the length of the comb and then on top of the existing locks, until the comb is about 1/3 full.
Hold your left hand stationary with the filled comb tines up. Take the other comb in your right hand and, just catching the ends of the locks at the top of the base comb, hold the comb so its tines are at right angles to the base combs, and comb away from your body with a circular motion to your right. This circular motion makes sure you have a complete pass of the comb without entering into the locks on the comb too deeply. You should feel a slight catch-and-release as you sweep the comb to, through, and away from the locks on the comb. Some of the locks will transfer to the right-hand comb. Repeat this motion, combing further and further down the locks with each pass. When most of the fiber has been transferred to the right hand comb, you have completed one pass of the combs.
There are GREAT YouTube videos by Rexenne on combing. Here's part 1 ...
Now, take your left-hand comb and decide if the "dregs" ought to be removed into your felting basket or not -- usually, they are for that or the trash can. Take your right-hand comb, newly filled, and put it in your left hand. Use the other, emptier comb in your right hand now to do another pass.
It usually takes three passes to comb your fibers. You can make additional passes if you want to ensure you have combed down to the most regular fiber length in your fiber.
And Rexenne continues with part 2 ...
To remove the fiber, take the final filled comb and rest it on a table with a book or heavy object over the handle. Start at one side of the comb, pinch some of the fibers and draw them out a short distance. Pinch the base of the pulled-out fibers and pull that out a short distance; this is similar to pre-drafting roving. Continue with short drafting motions, working from one side of the comb to the other and back again until only the final "dregs" are left in the comb.
You can wind up the drafted fiber into a birds nest by wrapping it around your hand and then pulling the end into the center, or if you want, you can spin directly from the comb by spinning as you draft fiber from it.
And Rexenne wraps up with part 3 ...
Final Notes ...
If you compare hand-carded roving to combed top, you will notice that the combed yarn has higher luster. This is because all of the fibers were spun parallel, increasing the reflectivity of the yarn. The trade-off tends to be elasticity; carded fiber has more air trapped in the yarn, permitting greater flexibility in the resulting yarn.
For those who like a joke, here's a funny YouTube offering also by the wonderful Rexenne: The Wool Comb Monster.
(from an inquiry on spinfree, 19 April 2007)