The raw materials: Starting bottom left...
Rubberised horsehair sheet - used in the restoration of old furniture. Find a self-employed restorer and ask him/her nicely for some offcuts of matting. If you're lucky you'll get some for free...and possibly an interesting conversation if you take along a T gauge loco!

A selection of scenery flock in suitable colours.
Diluted white glue - the usual formula with a drop of washing up liquid to break suface tension. You'll need some neat glue as well!
(optional...but useful) A syringe - to apply the glue. Mine was from a printer ink refill kit (its too big for using on myself...just in case you might be thinking!!

Cocktail sticks or toothpicks, etc.
Not in the picture.....brown paint for painting the trunks the appropriate colour.