It really isn't hard to do at all! It just takes some time.. And if you terrarium isn't 6 fet tall like mine that might not even be too much of a problem either
All you have to do is:
buy 2-4 small pieces of driftwood and some schredded coconut at your local pet shop/terrarium shupply.
Buy 3 or 4 cans of PUR foam and some tubes of black silicone at the hardware store (if you plan on keeping reptiles you might want to go for the silicone without fungicide)
That's the hard part...
Now just put your tank on the side, place the driftwood in some random places.. Spray a layer of foam around them to cover the entire back wall.
When dry cut the foam you don't want away
apply silicone in small patches and dap shredded coconut on..
Done..
Just go for it, you'll be glad you did!