Simply put, you will have a line(s) going from the tank to the sump. You can either use an overflow or a drilled tank hole. Then the water does its thing in the sump and goes back to the tank via a return pump with hosing attached.
Heres another picture. Hose on the left is coming from the tank ( I have a drilled tank), it goes into the sump and the water flows right until it gets to the return pump which is the hose setup on the right.
Hope this helps a bit!