The full instructions and list of materials needed can be found at west elm’s blog, Front & Main. I also talked about how to care for your terrarium, and what I would have done differently.
You could simplify the process by choosing a smaller container and just placing moss inside (like this one) with a few interesting rocks or objects. Of course, you could make it more complex too, by choosing an extra wide vessel and combining more plants of different sizes and textures.
I’d love to try a succulent terrarium next time, switching out the rich potting soil for a sandier blend, and using an open-air container. I think it would be fun to gather some HO scale model items to make a little scene, too. I do have a special fondness for miniatures, after all.