The next step is to add moss on top of the stones and charcoal. This is to keep your next layer, potting soil, from mixing in with the charcoal and stones. This is an aesthetic decision so this step is optional. I think it’s worth doing, because it can add visual interest to your terrarium.
With a large spoon or small shovel, add sterile soil-less potting mix on top of your moss or if you aren't using moss, put the potting mix right on top of your charcoal. Add as much potting mix as you can - at least a couple of inches. You will want to make sure your plants will fit into your terrarium with the top on, so you might have to hold your plants up to measure as you add soil.
At this point it’s important to think about the design of your terrarium. Will the terrarium have a back and a front? If so, you probably want to put your tallest plant in back, or in the middle. You can also contour your soil so that it mounds and sinks to create interest.