There have been numerous threads over in the equipment forum with design ideas...I'd suggest a search here on this site as well as via goggle or another search engine for additional info....I'm sure a few searchs will provide a ton of very psecific info. The basic idea is this:
First you need to have a means of removing water from the display to the refugium such as a hang on type overflow or a built in overflow...this will move water from the display to the refugium...then you will need a pump that moves slightly less water per hour than your overflow is rated for to pump water back to the display. The actual plumbing can be done with rigid PVC pipe or even flexiable tubing of some sort...but the general concept is to remove water via a overflow and return water via a pump.....not a very specific response but hopefully it'll get you pointed in the right direction.