It's odd stuff - it smells of rubber (given the name, this is hardly surprising), it's coarse-feeling, a little stiff, and in its raw form is quite dense (it's designed as an upholstery-stuffing material).
The end result is a tree with a core of plastic branches with a lacy mesh of rubberised horsehair around it.
Next, you take a nice big empty ice-cream tub and a big jar of coarse-ish flock of whatever colour takes your fancy, and spread out some newspaper, 'cause this next bit could get a little messy. Pour the flock into the tub, and then go find your big can of unscented hairspray.
As I mentioned before, this stuff is sticky. Take your tree armature, and liberally (and I mean liberally) spray it with hairspray. Dip the armature in the tub of flock, shake off the excess. Repeat. At least twice more. Certainly keep going with hairspray then flock until you're happy with how the tree looks.
One very important note: do not, at this point, do as I did and drop the tree. The hairspray is only a fairly tenuous hold, and you will get a fair bit of slightly tacky flock all over your jeans and floor. Don't do this. :D
Eventually, you'll wind up with something that looks like a tree. In fact, it looks better than the ones made with coarser clump foliage, in my book.
The next (and penultimate) stage is to liberally (again, I really do mean this) spray the end result with slightly dilute PVA until every bit of the foliage is coated, and put the tree someplace to dry for a good long time. The glue I used was Treemendus Scenefix Glue, which comes with its own spray atomiser, and unlike the Woodland Scenics one, seems to work. One tip though - when you're done, remove the spray head, hold the end of the tube in some running hot water and spray through for a while, so the drying glue doesn't clog up the works. (By the way, Treemendus are now promoted to my links sidebar for being fantastic - not only do they produce a PVA spray that works, they also make the awesome forest floor scatter I use.)
This is where I am at the moment. The final stage is to give it a bit of a haircut where (as you can see) some of the horsehair strands have got a bit straggly, but I'm going for patience here and leaving it to dry for at least a day. But I hope you can see at this point what the end result is going to come out like.