Thought i'd include a simple step by step on a basic diorama ...so ....
Start off with a picture frame. take out the glass and remove hooks and stand from the backing board. i've put a bit of polystyrene tile on top and glued the back board tight against the poly for strength.
I use a mini blow-torch to texture the surface of the poly and make tyre track divets etc.
I paint the surface with acrylics in the colour of whats going to be added... this give me an idea on the eventual look and the paint will show throught some of the scenic stuff and add a bit of body to the dio.
Made a tree out of twisted wire, which is covered in PVA and then bi-carb is sprinkled on. this is repeated untill it looks like bark and the twists from the wire have dissapeared. the bicard is then coloured with thinned paint, the bicarb soaks it up readily and you use slightly different shades to achieve a graduated/varied look for the bark. the foliage is scattter just sprinkled on the branched over a coat of PVA, again this is repeated untill it looks how you want it.
Added the tree to the dio by squishing it into some silicon sealant, th surface of the sealant has some grass scattered over it to blend it in to the grass already on the dio .. the roots are scraped clear of grass to give a little realism.
For information, the bull-rush type plants are made from a single bristle from a wallpaper brush, with a small amount of plasticene for the tip (painted brown) this is set in some garden twine to simulate the wispy grass.