Here are
directions to one many people have done. There are a lot of variations to it. Personally I found the conduit fittings a bit tricky and went with just Ts and think it works fine. I have 5 of this style on my ponds all a little differnt andall work well.
A shop vac is an easy alternative to a blower for cleaning too, its what makes the filter worthwhile.
The Sand traps a ton of fines like a pool filter but wont clog since its not pressurized. Banding/strapping also works well for bio and larger mechanical if you don't like the rocks idea. I have some
pics of some I built in this thread. Since I am in the south I went with a pipe down from the top to feed the filter rather than a shower drain on all but one. They are buried so in the winter I keep them full to keep the ground from crushing the barells.