There are some at OG but there is really nothing to it.

You get 4 furnace filters not cheep ones and not expensive ones. Get the ones that cost about 5 bucks at HD. They have a metal screen on one side to add support. Place the first one with the metal down. Add 2 pounds of active charcoal and spread it around (I get mine from the fish store that sells pond stuff it cost 4 bucks a pound). Then you place another filter (screen side down) add another 2-pound bag of charcoal then the third filter and the last 2 pound bag of charcoal. Last you want to place the last filter with the metal screen side up. Line them all up and use gray tape to seal all the ends together so the air can only go through the filter and not lose any charcoal out the ends. Make a box that all 4 filters fit snugly in. At the top of the box leave about 8ā? of space. Install your fart fan at the top of your box the 8ā? spacing will help to even out the air pressure on the intake. Now I use a furnace screen like a house uses slightly larger then the filters so with 4 screws I can change it out and access what I need to. Mine is installed in the roof and the return just pumps it back into my room. Really itā??s that easy the whole thing is to seal all the filters together so they are just one thick filter and to have the air gap at the top so your fart fan does not just draw air from one place in the filter. It really does work.

Peace