Add a 1- to 2-inch layer of pebbles to the bottom of the container for drainage (image 1).
Add a 1-1/2-inch layer of activated charcoal over the pebbles (image 2).
Add a 1-1/2-inch layer of sphagnum moss over the charcoal. This will help prevent soil from moving down between the pebbles.
Pre-moisten the potting soil until damp but not soggy, and add three to four inches to the bottom of the container. The amount you add will depend on the size of the container and the type and size of plants that you want to grow.