Use a marker to mark a point around an inch above the bottom. Use sharp razor or scissors to cut off the bottom from that point. Be careful about you hands while cutting out the bottle as the edges can be sharp. If you are a kid enthusiast ask a parent to help you with this part.
Sink the cut edge well into the ground. You can also dig a bit to make it stand stronger in case the wind is hard enough to blow it away. Unscrew the cap. This will allow ventilation for the plant and vent it off when it gets too humid.
You have set up an excellent protection barrier around your plant that also facilitates its rapid growth. Even if your plant outgrows the bottle, cut the top off the bottle and leave a small section around the plant. This will save it from over stepping accidents.