First, cut the plastic bottle from the waist in the middle area, remove the cap, and tie a piece of less dense cloth firmly to the mouth of the bottle. Then place the mouth of the bottle upside down inside the other half of the bottle to make it look like a funnel.
Next, fill the funnel with a layer of sand. If possible, add a layer of activated carbon, then add gravel, and finally some pebbles.
A simple water filter is completed.