First you will need to get proper hole saw and the bulkheads you want to use. YouTube has a few aquarium hole drilling videos to follow.
I bought my stuff from glass-holes.com. There are other places as well. Anyway glass-holes also has PDF directions that you can download and read before deciding on doing this. It took me about 15 minutes to drill 4 holes into the back of my 75.
I just watched a video made by diy king or something like that. Took him 30 seconds. Fairly simple. I think ill do that. Glass holes has ridiculous prices to be honest. But I do need some help. Im gonna use the mag 12 but should I have 2 holes or just 1? Its gonna be 1 1/2"