i just made one this afternoon, it's incredibly easy!
take the sponge (the one i used is about an inch wide, so that's the frame of reference i'm going by) cut it into sections so it's relatively cubic (i did mine in quarters so it's 4 inches by 4 inches by 4 inches)
cut off about 6 inches of piping (i used the hose from a gravel vacuum, we're getting a new one tomorrow) take a knife, and cut a hole straight through all but the bottom piece on the stack. take the bottom piece, cut roughly 1/2 of the way through it.
push your piping all the way through the stack, till it's nestled in the half-hole at the bottom. take three zip-ties and push the end through the whole stack, then zip them together at three points to keep it together.
take an aquarium-safe rock or other small, heavy object and silicone it to the bottom of the filter. this will give it negative buoyancy and keep it in place in your tank. allow 24-48 hours to dry fully.
take a length of airline, a small-sized airstone and a pump, attach the stone to one end and the pump to the other. thread the stone and airline down to the bottom of the piping and plug the pump into an outlet.
it should now be pumping air down into the filter, forcing water in through the bottom and out the piping at the top. congratulations, you've made a sponge filter.