Cutting a hole in an angled surface Ok, so lets say I have a mesh like the one on the left, and I want to make a hole in it like the one on the right, as illustrated here:



but, using the methods I can think of, you end up with  inner vertices that are out of alignment with the outer edge, thus producing a non-flat surface as you can kind of see here:



also, as I just eyeballed it, the shape and curves of the hole are not totally symmetrical at each corner, so, what is the best way to cut a hole like this, with symmetrical measurements, in a surface that is at an angle, and keeping everything in perfect alignment with the surface itself without creating any n-gons and having clean topology?
Maybe you should use boolean modyfier. 


Create an object that has the shape of the hole you want to create.

Add boolean modyfier to the object you want to cut a hole into.

Make sure that the operation type was set to difference. Choose object that you want to use as holecutter.





Slide the holecutter into the place where you want hole to be.

Apply the modyfier and remove the holecutter.

Hope you understood my bad english, but if this post helped then let me know by votng this answer up. Cheers! :)