How to avoid cutting electrical wires when cutting into drywal
First you need to lay out the general area where you want the opening. It needs to fit between 24" centered ceiling joists.

Properly installed electric cabling is always at least 1 1/2" away from the back edge of the sheetrock (or protected by a metal shield). So, if you cut a small hole near the middle of your proposed opening with a shallow cutting tool, such as a utility knife or a multisaw, big enough to get your hand and arm through, you can reach in to make sure there are no wires draped in the area where you are working. If clear, cut a bigger hole with a drywall saw or multitool, large enough to get your head through. Insert a light and then your head and scan the area. 

Now you're prepared to cut the full opening back to the joist with no surprises.