Cut the top off a plastic barrel with a knife and a hacksaw. Drill a half-inch drainage hole in the center of the bottom of the barrel.
Line the bottom of the barrel with a cotton sheet.
Set the barrel on a stand. Use whatever works for the stand, such as sawhorses, cinder blocks or an old pallet, but remember you need to place your water storage container(s) beneath it.
Shovel sand into the barrel, filling the bottom with an even 2-inch-thick layer.
Shovel charcoal into the barrel until you create a 4-inch-thick layer. Periodically smash the charcoal with the shovel, breaking up any large chunks and ensuring you have a compact layer.
Fill the barrel with alternating layers of sand and charcoal, repeating Steps 3 and 4, until you have filled the barrel to roughly three-quarters from the top. Make the last layer sand, even if it means making the charcoal layer beneath it thinner. This creates a reservoir for pouring water into the filter.
Place one hose from your drainage pump into the pool, and the other hose into the reservoir at the top of the barrel filter. Put your first storage container underneath the stand. Turn the pump on.
Adjust the speed of the pump so it roughly matches the speed at which the filter processes the water as closely as possible, favoring the pump working more slowly if necessary. Whenever a storage container is full, replace it.