I really like using a spiral saw for drywall repairs and especially cutting recepticle holes. With a drywall bit, you can trace the outline of an electric box, or follow a framing member. You can cut freehand as well. A manual drywall saw is also very effective. Of course when cutting sheets, use a straightedge and razor knife to score the finished side, break the board along the line and cut off with the knife.
Not sure how your repair is being done, I have often been able to remove the old box, then used a piece of scrap drywall as a backing patch (glue on front of patch, feed behind drywall and pull forward) then filled with a second piece of drywall cut to fit and fill with compound. The patches sold in paint and hardware stores work well. A thin metal plate is stuck on the hole and the whole patch is reinforced with fiberglass tape.
If you have more specific questions, please feel free to post.