Otherwise, it's very easy to miss the framing in the middle of the panels. Don't let the butt joints at the short ends of the panels fall on the same joists or studs from row to row. To make a long, straight drywall cut, score the front side of the panel by running a utility knife along a drywall T-square, snap the end of the panel away from the scored line to break the gypsum and then cut the paper backing at the break line to finish the cut. Use a compass to make circles. Punch through the waste area with a keyhole saw and cut along the outline.
Test fit the panel and trim the hole as needed with a drywall rasp.
Once the drywall is in place, any new soffits can be framed and covered with drywall also. Their usual function is to fill in the open spaces between the ceiling and the tops of the wall cabinets, but they can also be used to hide new plumbing pipes if those are roughed-in outside the wall. The front is usually built by suspending a short wall from the ceiling, and the bottom is framed between that and the wall. A pair of 2x4 nailing strips are secured first, one to the ceiling and one to the wall but running parallel to each other. The top of the framing ladder is set flush to the ceiling, snug against the edge of the nailing strip, and it's nailed or screwed into it to secure it solidly.