Turboflotor and a lot of other skimmer manufacturers advise a "T" off the main overflow drain to feed the skimmer, or a pump which feeds from the sump. Use a gate or ball valve just before the skimmer to prevent running too large a flow through skimmer. All skimmer manufactures usually advise a certain flow rate for their skimmers. Use the "T" with an input at top, the line to the skimmer at the bottom and the third branch off to the side to run to the sump. This will assure a steady head of water against the regulating valve controlling the flow into the skimmer. I think that was a simple explanation. The skimmer output goes to the sump. Most people do not run all the water going to their sump through a skimmer in a single pass as that would require a larger skimmer than most people would choose to purchase.