You'll need to provide good drainage so that your plants don't become waterlogged, which can lead to root rot and yellowing leaves. Pour a little horticultural grit or some small, clean pebbles into the bottom of your bottle so that excess water will pool there instead of around your plants' roots. You can buy horticultural grit or planting gravel online or from your local garden supply center. The type of potting soil you choose will depend on the level of moisture and the types of nutrients your plants require. In general, though, it will need to be well-draining, high in organic content, and free of contaminants that might cause infections in your plants. Research the kind of potting soil that's best for the plants you plan to grow and buy an appropriate mixture at your local garden center. For example, for succulents, try purchasing a fast-draining potting soil designed for cacti. For more moisture-loving plants, such as peace lilies, opt for a general-purpose houseplant soil that holds a little moisture while still draining well. Pour the soil into the bottle on top of the horticultural grit. Break up any lumps with your hands so that the soil is loose and airy. It can be helpful to moisten the soil slightly before you pour it in to prevent the sides of the bottle from getting dusty.

Summary:
Add 2 inches (5.1cm) of horticultural grit to the bottom of the bottle. Purchase a potting soil that's appropriate for your plants. Put in a layer of potting soil until the bottle is 1/3 full.