By placing that tank at a level lower than your main tank, installing a drain or external overflow in your aquarium that flows into said tank, and having a pump to return the water to the main tank. It is that easy. Turning a sump into an efficient filter system is a completely different kettle of fish, and if that is what you want to do I'd suggest you start searching through the DIY Filters forum (and this one) to see how other people have theirs set up so you can get some ideas about the different options for mechanical and biological filtration and figure out what will work for you.