For drainage purposes, I placed a thin layer of grit in the bottom of the terrarium. Then using the potting compost I filled it until it was almost level with the opening. On a lot of the tutorials I kept seeing that activated charcoal was used, but it's only if you have a sealed terrarium that it is necessary (it helps keep mould at bay). Then I took each of my plants and played around with where I wanted them to sit. It's a tight space to work with, so make sure you put the ones at the back in first when you come to actual planting. I made this mistake and learned the hard way! Once in place I gave each one a really good water. A thin layer of moss around each succulent and it was done.