Do your research first on what type of overflow you want. There are a bunch of different ways to do this. My recommendation would be to go with a low profile overflow box, something like the modular marine or stealthboxes. For these, the holes are drilled up high, but you definitely want to have the overflow box in hand before you start as you need it to set your water height and hole spacing.
I'd also highly recommend looking into the 3 standpipe "beananimal" overflow styles. I cant stand aquarium noise, and these can be tuned to be almost silent and have a couple fail-safes in case of pluggage.
I'd also highly recommend looking into the 3 standpipe "beananimal" overflow styles. I cant stand aquarium noise, and these can be tuned to be almost silent and have a couple fail-safes in case of pluggage.