This is the sump mine's based on. I only have 2 chambers instead of 4. I skipped the bioballs chamber, added a spray bar and divided the 20g tank the long way instead of the short way.
My plumbing is based on
Inland Reef's DIY guide only kinda backwards.
I know this is long and I tried to overly simplify it, but I felt I had to in order to describe it with out a pic.
What I did was take plexiglass and cut it to fit the length of a 20g. It's also 2-3 inches shorter than the top of the tank. On the right side I cut a little notch in the top of what is to be the plexiglass wall - about 1 inch deep by 4 inches long. This is will the water will flow from the refugium half into the sump half. I fastened it into the 20g tank - making the refugium the front half and the sump the back half so I could see the refugium - neat stuff!
For the plumbing, I don't have a predrilled tank (I definitely will nest time - what a hassle!) so I have an overflow bow on the back of my main tank. I cut 1 inch (I believe - it's been a while and it's downstairs
) pvc to go from the bottom of the overflow to the top of the refugium. I put an elbow on that and attached a straight length of pvc, only a couple inches long to go into a T fitting. The bottom part of the T fitting (what would be the vertical part if you were holdin it like a T) is connected to a hose and drains into the sump.
I put another short length of PVC on the other end of the T and put a ball valve on that. Then another short length, an elbow and a then a spray bar - drilled PVC - 1/8th inch holes I believe. The spray bar is what gets water to the refugium.
The ball valve is open all the way, but I like it there so I can turn the refugium off to get a good look down into it or in case I had to do anything to it for any reason.
I control the amount of water going into the sump versus the refugium by angling the T valve and hose.