I start by drilling a series of holes in some scrap timber. The size and depth of the holes will control the amount of splay on the branches.
The MDF doesn't drill very cleanly, so I had to clean up all the holes with a counter-sink.
After that, I filled the holes with carpenter's glue, and then plugged each one with some "branches" cut from a wallpaper brush.
After the glue dried, I trimmed the bushes into pleasing shapes with scissors.
Next it was time to flock the bushes with the finer branch detail. In the picture I'm fogging them with 1:4 matte medium:water with an automotive touch-up gun. A misting spray bottle will work too.
I flocked them with golden static grass fibre so I wouldn't have to paint them afterwards.
After that dried, they got another diluted matte medium application followed by flocking with spring-green SuperLeaf. You could also use fine-ground foam (the leaf is slightly over-size for HO, but would likely work well in S or O).
I always give them one more coat of diluted matte medium at the end, and then cut them off the board with an X-acto knife.
Cheers,
Jeff.