You probably wont find anything useful Googling it. A overflow is simple, but depending on where you need/want place it in your system, and what materials you plan to use (vinyl tubing/PVC tubing etc.), the type of system your building (NFT, Flood & drain etc.) will determine the best way to do it. For instance if you want the line to go through a curved surface like the side of the tubing, that will present the problem of getting a flush fit without some type of curved rubber gasket or permanently gluing it in place. But if you place it in the end caps that have a straight wall, it's easy to get a flush fit, as well as good seal with just an o-ring.
If you have any pictures or drawings of the system your building, and where you want the fill and overflow to go, we can help easily you with it. Here is a link to a system I built using the same ADS tubing:
Third system. You can see I went in through the end caps because that provided a flat surface. I didn't make the overflow adjustable in that system, but there are easy ways I could have. And you don't necessarily need to use the same bulkhead fittings I did. You can also use a male and female PVC adapter, and o-ring to make your own. In my experience I would suggest to make sure your overflow is at least twice the size as your fill line. Water goes in under pressure from the
pump, but only goes out under gravity.
You can always compensate by regulating the water pressure from the pump a couple ways, and may need to anyway. But I've learned to just make sure the overflow can handle the volume in the first place. It saves a lot of headaches. So if your water fill line is 1/2 inch. I would use a overflow of 1 inch, or two 1/2 inch outlets.