We constructed the full assembly of the bottle biome. First, after getting both bottle halves, we added gravel to the bottom with water after that. Then, we taped the top half to the bottom half to prevent air bubbles, as well as poking a hole in the cap with a string running though it. Eventually, we added soil and grass seed with a light watering on top to set up the rest of the interior. Finally, we capped it off with another bottle top and taped it on to ensure no air escapes.
Bottle Biome Project Blog 