This is a great project for those new to the vivarium construction hobby. It's fast, easy, and relatively cheap as builds go. This project isn't so much building a new background for your tank as it is resurfacing a preexisting one. A new exo-terra background could be purchased and cut down to fit a variety of tanks but the project works best for exo-terra (or similar terrariums). This tutorial is perfect for someone with an older exo-terra terrarium and still has the original styrofoam background (in mint condition or damaged).
Larger reptiles tend to do a fair bit of damage to these exo-terra backgrounds after months of climibing, scratching and gnawing and since the majority of exo-terra's backgrounds don't look as nice as this:

I have developed a way to resurface them to make them more realistic, and hard as rock.
The process I follow is one that is pretty common in the vivarium hobby-world used to create faux rock.
For this project you will need:
- exo-terra background that fits your terrarium
- unmixed sanded grout
- paint brushes
- earthy tone acrylic paints (white, black, browns and yellows)
- mist bottle
- spray-on, non-toxic acrylic sealer
Now we can begin.
The first step is to make sure your styrofoam background is well cleaned. You can use an old toothbrush and some dish soap to quickly scrub down the background. rinse well afterward.
Next it's grout mixing time. beginners may have a tricky time with this, but read the directions on your sanded grout. As you mix your grout, you should aim for approximately the consistency of pancake batter. If you find your mix is too thin, add more grout; too thick, add a small amount of water.
I prefer to use a foam brush like this to apply the grout:
Now you can begin applying the grout covering the entire frontside of the background. Do not worry if the grout runs off the sides, you will remove it before the painting step. It is not important to worry about the texture of the grout for the first two layers but make sure not to completely fill preexisting valleys in the styrofoam background.
You will need to apply between three and four layers of grout to your styrofoam background. It is important to let your grout dry completely between layers. Give it a full day to set when possible. Below are some pictures from the grouting process.
As you reach your final layer, you can continue to brush the grout onto the background but I find this technique creates unnatural looking streaks. Often when I reach this step I will apply the grout and then use my brush to dab it creating a more natural looking finish to my background.
After your final layer of grout you should let the background sit in a well ventilated area for at least two days to ensure the grout sets properly. The next step is to prepare it for painting. It is likely that grout has dribbled over the sides of your background and hardened. At this time you can take the paint stick you use to mix your grout and scrape it across the edges of your background (sides top and bottom).
If you do not do this step now, there is a chance your background will no longer fit into your tank, and you could damage it trying to force it to fit.
Here comes the creative and artistic part... painting. I begin by applying a neutral base coat of either grey or brown. I often thin my base coat acrylic paint with water in a mist bottle for quick application. Your background should look similar to the below photographs at this point.
Continue by gently brushing different earthy tones onto your background, the rough texture of the grout will assist in making natural looking layers of colour. If you make a mistake any time during the painting process, don't worry. You can just paint over your errors

. The following pictures show the layering process.
Once you are satisfied with the look of your painted background, it should look similar to this.
You are now ready to seal the background. Follow the directions on your non-toxic acrylic sealer and apply approximately 3 even coats, providing time to dry between applications. You should apply the sealant to the top bottom and sides as well to ensure a full barrier.
YOU'RE DONE! But you should let the background air out in a ventilated area for approximately a week to let the fumes from the sealant subside.
Below are some examples of completed backgrounds.
And here are some photographs of these backgrounds in use inside terrariums.
Hope you all enjoy this project and feel free to ask me any questions you may have.