SerpaDesign teaches us all the steps needed to create a bottle terrarium, even in a bottle with a narrow neck. This technique focuses on plants rather than striking hardscape.
The key to a successful closed terrarium is something called a riparian layer, or “false bottom”. This consists of a layer of gravel covered by a thin mesh — a piece of window screen will do nicely — that allows water to separate from the substrate and seep into the gravel below.
In order to get the false bottom into the bottle, use a funnel to pour the gravel. Then use a pair of long tweezers to place the folded mech circle on top of the gravel, unfolding it inside. On top of this add some charcoal, your substrate of choice, and then you’re ready to plant.