A sump / refugium can be as simple as a tank with plates of glass siliconed in. Generally, there are 3 areas to it.
First piece is where the water from the display drains into. This is where the skimmer usually sits and where a bubble trap is as well. This only needs to be large enough to fit a skimmer into. The bubble trap is a series of 3 panes of glass approximately a half an inch apart. The First and 3rd panes of glass should be the same height and they will regulate the depth of the water in the first chamber. The middle pane of glass needs to be raised approximately an inch off the bottom to force the water to flow under.
Second is the refugium section. This is the bread and butter of the build. Just a large space to grow algae in. Make this section as large as possible.
The last section is the return pump. Just a single pane of glass that will regulate the depth of the water in the refugium section. Make the last section large enough to hold the return pump + 2 to 3 inches.
It's a lot of text but it's a really simple thing.
This picture demonstrates it quite well. They added an extra baffle to prevent any bubbles from getting to the pump. Not necessary, but it works.
As to getting the water out of your tank and into the display, a drilled tank is the way to go. You can usually get a glass place to do this for you or a small fee. A 1.5 inch hole is fairly standard and should work for most applications. You will need a bulkhead and a baffle box for this. Drill the hole near the top of the tank, insert the bulkhead, and then attach the baffles.
plumbing will need to be ran to the refugium. It can be a bit of a pain to get to work but just try it out dry fitting it before gluing it down.
The return pump, shoot for something that will give you at minimum 10x the volume of the refugium / sump per hour. Keep in mind that height will decrease the flow of the pump.