Well I too have asked this question.
It looks like you have a nice "base" plan. I'll add a few tips for ya.
On the refugium in-let I suggest that you feed it from the main tank over-flow line, just add a "Y" fitting to it and run a line from that to the refugium. You should add a ball valve at the end so that you can regulate the GPH going into the ref.(I would suggest using 3/4" line and valve for the ref. line)
Then for the out-let side of the refugium just allow the water to over flow over the divider back into the sump compartment.
Also from what I've been told you may want
to increase the GPH rate of the return pump up to bout 800-1000 GPH(just make sure that the main over-flow line can handle that much flow.
Other then that it looks like you should be on your way to a nice looking DIY sump/refugium. Don't forget to take LOTS of pictures