I can only show you how I did mine ... there as many "correct" ways to do it.
The overflow box is centered along the back wall of my (new) 100g tank:
Two 1.5 inch bulkheads for drains, Two 1" bulkheads for return(s).
Here's after it was plumbed to the sump:
drains into the left side of the sump, with the skimmer... through a bubble-trap into the return section (center) ... an extra line t'd from the main drain goes to the refuge on the left (with a valve to control flow.)
(This pic was before I added the return line within the tank/overflow... but that's just a section of PVC
with a T and two Loc-Line return nozzles into the main part of the tank.)
The 4th hole I drilled is just capped off ... not being used, but might come in handy in the future.
Filled and cycling:
OK, here's one with the return lines plumbed: