Assemble your materials.
Pour sand into terrarium. Give container a shake or two to level sand.
Loosen moss clump, without actually separating pieces. Drop loosened moss clump down into container and arrange so fluffy pieces are facing up and out.
Form a hole in moss toward the center for potted plant.
Add small pieces of reindeer moss.
Arrange rocks by sliding them down the sides of the terrarium.
Add sea urchins against glass. Slip potted plant into hole in center of moss.
Add remaining reindeer moss to conceal pot. Voila—done!