I have some pictures I'll look for on where to drill the first holes. I'll go hunt for it and see if I can put it on here.
The main thing is: to double and triple check everything measurement wise before you dedicate any holes or cutting.
What you do is on the back center; you drill 2 holes side by side. Then you sawzall them together into one. This is where you begin to do the cut around following the tape you layed out as the template.
The outer edge of the hole of course will fall within the cut edge so it doesn't go out. The ring covers any areas you goofed regardless. On all 4 corners, bring your tape line in just a little. The reason is: because you only have one chance. you don't want to end up that the thru studs on the outer ring the drops in, doesn't even have anywhere to go thru. You can re sawzall whatever is too much later on.
I would recommend reading this article. If after this you need any further info, contact me.
http://www.moparmusclemagazine.com/techarticles/mopp_0710_shaker_hood_scoop_install/index.html