Here's what I did with my woodland scenics flock (NOTE: this might not achieve the effect you want). I have to admit, the process I used is long and not guaranteed to work. So, probably ignore this whole post.
First, I outlined the area that I wanted to flock with a bead of Elmer's glue (white PVA glue). Then a took some watered-down Elmer's glue (not watered-down so much that it was watery, just enought that it didn't dry out while I was spreading it) and 'painted' the area I wanted flock. Then, after there was glue everywhere I wanted flock, I sifted flock over the gluey areas.
I let that sit overnight.
The next day, I took the flocked surfaces outside and shook off all the excess flock. After this, the surfaces looked rather ratty, so...
Using even more watered-down glue (now watered-down to the the consistency of milk) I carefully soaked the semi-flocked areas (the flocked areas act as a sponge, so it's not that hard). Once those areas are sopping wet with glue, I spread even more flock on top. Waiting for 5 minutes will reveal wet spots soaking through in some areas, so sprinkle on more flock where they appear. Then walk away for an hour or so.
Now, it takes a whole lot longer for this mess to dry than an hour, but... After an hour or 2 things should have dried off enough for you to be able to break out the rolling pin. Use the rolling to roll the surface smooth, but DO NOT roll over the same area twice (clumps of gluey flock will attach to the roller and ruin all that work).
Repeat steps as necessary.
In the end, I don't think I'll be using flock again. It's too much hassle. Instead I am planning on experimenting with spray-on wall texture (the finest grit size), ink, dry brushing, and a matte sealant. I just wish they made
this stuff in grassy greens and tans. I've used this stuff to make ashe wastes and to make tree trunks look like they've been burned in a fire, and it works amazingly well.