You'll need either a knife or scissors. Cut the bottle in half.
Step 2: Place the Halves Together.
Keep the bottom part upright. Take the other half (the half of the bottle where the cap goes) and cover the top with a coffee filter and then place it inside the other half. Tie it in place with a rubber band. Make sure to press down somewhat to secure the halves.
Step 3: Add Charcoal
Step 4: Add Aquarium Pebbles
Step 5: Add Gravel
Step 6: Add Charcoal Again
Step 7: Add Aquarium Pebbles Again
Step 8: Add Gravel Again
Step 9: Repeat Until There Is 1/4th Empty Space Left on the Top Bottle