Yesterday I finally purchased the ideal terrarium for my build (Exo-Terra Medium Tall, 24" x 18" x 24") as well as some fixings for the background, and I'm ready to start. I'll be making a custom background with live plants for the terrarium, so the first step is throwing away the junk foam background.
Now I'll need to lay the groundwork for the background. Before anything else can happen the background needs to be set and cured, so it's important to start on that early. I'll first be putting a small layer of silicone on the back, which will allow the Great Stuff I'll be applying later to stick better.
I'm using Aqueon Silicone Clear for this job. This step isn't 100% necessary, but I felt it was important to guarantee a good sticking background. I use a putty knife to spread out the excess and give it a nice even covering.
The whole process took about 10 minutes; the clear silicone means mistakes are much less noticeable and aesthetics are not important here.
Now, we let it sit for a day or two to cure properly. Next step is the fun part - foaming the background.