Cut away the bottom of one bottle using a pocket knife or utility knife.
Turn the plastic bottle upside down. Fill the neck of the bottle with cotton. Use pebbles if you do not have any cotton in your first aid kit. You can use a 16-ounce bottle, but a 64-ounce (half-gallon) bottle or other larger container would be more efficient.
Pour about an inch of fine sand into the bottle on top of the cotton. If you have activated charcoal, pour a layer about an inch deep into the bottle on top of the fine sand. Now pour in a layer of coarse sand, followed by a layer of fine gravel and a layer of coarse gravel, making each layer fill about one-third of the bottle.
Place the second container under the neck of the container that is now your filter. Pour the dirty water into the top of the filter and collect the filtered water in the container below.
