Percolators or "percs" are simply another method for further filtration/cooling of smoke. Most common styles are the tree perc and dome perc and rather than trying to explain them in text, you can see easily how they work on youtube or something of the like.
If you use the search feature at the top you will find all sorts of info on how to make homemades. I have seen a tree perc with a pill bottle and straws.
your easiest way to "make a perc" would be a dome style perc. take a pill bottle, drill holes near the opening. run a tube through the lid. fill w/ water (outside of the perc) and wham basic dome style perc. smaller holes and the more holes the more diffusion you get... if you go to small you start getting alot of drag so just play around w/ it til ya find something that works for you.