(these dimensions are for a 5 gallon tank, adjust as needed) here's what you need:
2 chemical-free kitchen sponges
5 feet of 1/4 inch diameter airline tubing
5 inches of 1 inch diameter tubing (a section from an old gravel siphon works great)
1 small airstone
1 aquarium air pump rated for 5-15 gallon aeration
1 flow adjustment valve
1 4-inch diameter terra cotta pot
3-5 live stem plants
3 zip ties
3 small aquarium rocks or glass marbles
1 plastic ring about 3-4 inches wide (linking toys work great)
x-acto knife or nail fine
dremel or rotary sander
rinse the pot out thoroughly. take the rotary sander, attach a bit on it that will fit in the hole in the bottom of the pot. begin sanding the hole till the whole bottom of the pot is gone.
take your sponges and scissors, cut them into circles about 3-4 inches wide. sandwich your rocks/marbles in between the sections and push the zip ties through the material to hold it together. the rocks will give it negative buoyancy and prevent it from floating/moving. cut the extra material off the zip ties so your fish don't get cut on them.
with your file or knife, make a 1 inch wide cut all the way through the center of the first sponge and 3/4 through the second. now make 3-5 quarter-inch additional cuts through the first sponge, all around the main cut.
slide your 1-inch tubing down into the center cut till it hits the material on the bottom.
(now would be a good time to install it, the rest will be easier to do already underwater)
take your bottomless pot and place it on top of the sponges with the tube coming out of the middle.
trim any leaves from the bottom 2 inches of your stem plants. gently press them into the quarter inch cuts till the ends are between the sponges.
cut about a foot of your airline off. install the adjuster valve between the two sections, attach the short end to the pump, and the long end to the airstone. thread the airstone and tube through the plastic ring and into the thicker tube and push it all the way down. the ring should float on the surface and baffle the current from the filter.
turn on the pump and adjust the flow as needed. you now have a working sponge filter that also serves as a decorative piece in your tank!