I'm no expert but an overflow box is two boxes that hang on your tank. One inside and one outside. You use "U" shaped tube to suck the water that "overflows into the box in your tank over the lip of your tank and into the next box. Yes water does have this strange ability to flow upwards as long as it goes downwards after. It's really something that a friend or your LFS has to show you. Anyway from there you use the PVC pipes and let the water flow out of the hole the overflow has, down into your sump. Now you need the sump for a reason, like a wet/dry filtration or refigium or something. Then you pump it back into your tank which raises your water level and hence "overflows" into your overflow box.
Be carefull about suction!!! The same power of liquids that can make water flow upwards is easily reversed. Hence if your return pump stops the water will flow backwards back into your sump. Leve the water level low enough rthat it can accommidate this factor. It will only stop flowing back into your sump when it sucks in air from the the first hole at the end of your return hose (the deeper in your tank you put this hole the more water will backflow).
Also a hard thing to understand to do is get the "U" tude started in your overflow. You need a thin flexable hose, then you put it up just past the highest point of the curve and start to suck the air out of the tude. This will draw the water up it and start the flow. Get all the air out or it will keep slowly getting air bubble to collect until the air pocket cover the whole curse of the "U" tube and water stops to flow. This can also happen when the "U" tube gets clugged.
Next problem. If these "U" flow stops then it is not returning back to the sump. So now you have all the water from your sump going back into your tank (which only has a inch or two from the lip) and will start to overflow onto your floor. The cure is to buy an auto shut-off. This is an electric cord that connects to you pump and float on your tank's water level. When it gets too high the it stops the electric current and stops your pump. This is a must have in my opinion.
This should be all you need to know. Now tehre is many ways to setup a sump and depending on what kind of filtration it's for. but here is one good method if you go the refigium way.
Goo luck and keep reading and search these subjects to see what others have been posting. Read, ask question, and read more before you do anything.