Drilling return?
 

It is my understanding that it's wise to drill a hole in your return to avoid a back siphon. Is there a specific place this needs to be done or just wherever under the water line?

It's a good idea if it's needed. If you can turn your equipment off and your sump can handle the extra water from the siphon then don't drill any break holes. If it can't, drill them just below water surface and at the side of the pipe if possible to avoid spraying water upwards. Be careful though as relying on such things isn't always a good thing as algae will clog them often which can lead to failure. Always best to design the system so such things aren't needed.

I was going to drill a hole as well but decided to move the pvc return up higher, closer to the water surface. It looked better and no need to worry about a back siphon.