I built one for Mockingbird a few years back. They're time consuming, but not terribly difficult.
The trunk was a 12' piece of
sonotube, roots were 2x4 scraps stacked up and screwed together and covered with
chicken wire and
canvas so Scout could stand on them.
I built a
conduit frame for each of the 3 main branches which split to several secondary branches which were bent in a natural looking curve. The junction of the tree and limbs were built from
chicken wire (god, how I loathe
chicken wire) and tapered down to pool noodles and great stuff which was carved to
taper down to bare
conduit. All of that was mus covered. Then I gathered limbs and attached them to the
conduit, because, like someone said in a previous comment, fiddling with all of those tiny branches isn't worth the time and likely won't turn out how you wanted. My branches were rigged to the catwalks above, so I didn't have to worry about the structure too much, and the trunk was screwed to a wall. I tried various methods to make the bark more realistic, and went with tissue paper. Rip the tissue paper into a bunch of strips and crumple them into a ball. Flatten it back out, still leaving it fairly crumpled and stack them up in a box. Paint a glue/water mixture on your branch or trunk and place a piece of tissue paper on it. Use more of the glue/water mixture to smooth it down, taking care to not lose too much texture. After that, just pick the
base color, highlights and shadows from the found limbs.
Base coat it and shadow, then highlight the texture with a
dry brush. Old scraps of towels that have been stapled on and painted green can help
cover the seam to make a good transition to the floor.