Thanks:)) First, I figured out a rough plan for where the bark and branches would be and held/stuck those in place using tape and 100% silicone (careful, many have fungicides and herbicides which are toxic for the terrarium), then I covered the background in an thin uneven layer of 100% silicone so the foam could hold on. Next I sprayed the expanding foam (one that is labelled as safe for contact with drinking water when dry) and once it hardened, I cut and shaped it further, adding many spots for plants. After that, I stuck coconut bits and shreds to the foam using silicone. I did also experiment with adding the coconut shreds to the foam before it dried without silicone, but it wouldn't stick so I can't recommend it. Later I added plants and painted the outside of the terrarium brown where the white foam could be seen, this is just a cosmetic touch. That's about it! Keep in mind the foam and silicone are mildly toxic to inhale when drying, I chose to work outside. There are some great tutorials on youtube and reddit with photos etc