Start by cleaning your empty habitat with a bit of water and paper. Then you will start to add silicone glue to all of the clean background. Make sure to cover all of the areas in which you will be spray foaming later. My suggestion for this part is to use transparent silicone so that if some silicone is spilled in another part of the terrarium, it will only blur the part a little, considering sometimes it can be hard to fully clean.
Please consider using silicone glue that is appropriate for glass and that the packaging clearly states 100% Silicone, and double-check that it does not contain any harmful chemicals or mold inhibitors that could be toxic to your animals.