First and foremost you need to know or decide what overflow you are going to use. Overflows are rated in gph that they can handle. Are you attempting to add one to a tank or does the tank have one already? If you are adding to the tank is the tank dry or running? Hang on back overflow scare me they fail pretty regularly. I wont use them. For a new tank without overflow there are a few manufactures of overflows that typically come with glass hole saws, eshopps, synergy, fiji cube, I have used Eshopps and they are pretty straight forward and reliable. Its pretty disheartening to find out the glass is not drillable after you start drilling so make sure the glass isnt tempered before you buy everything and ruin the tank. There are guides online to help figure this out but I would be "surprised" to see a tempered 40b. Most of the time a single hole through the glass will typically mean around 600 gph but since the hole through the glass is typically a larger diameter than the pipes that lead to the sump the number of pipes in the overflow box really dictate the flow rate. At any rate if the box is designed to handle up to 600 gph I would shoot for a return pump that will deliver 400-500 gph at the tank after head height. Head height is the distance from the pumps output to the tank "inlet" for water including any restriction from pipes including elbows and what not. I would not say this is a task for someone new to reef tanks or fish tanks in general. But to be honest if you are a diy type person or comfortable with working with tools, cutting wood, building stuff, plumbing, its not that difficult either, I learned by doing it myself.
I Googled a few things like, reef tank plumbing, how to install an overflow on a reef tank, how to plumb a sump for a fish tank, how to drill an aquarium, and found quite a few videos and walk throughs just now. Its not always easy to just tell someone how to do this, not everyone learns the same or teaches the same. I dont know your knowledge level or your "skill" level so I tried to make the above as easy to understand as I could. Was that over your head? IDK. Or was it too "easy" and not enough detail? IDK again. But I hope that helped somewhat. Try googling what I did and watch a few minutes or hours of youtube videos. Try starting at the point of how will you get the water out of the tank and to the sump probably the most important place to start since typically we want the water to stay in the aquarium.