These tools:
1)Roto zip with vacuum attachment for dust collection
2)Shop Vac to attach to the dust collector
3)1"x6" board the length of the cut(cull lumber works great for this)
4)Drop cloths
5)Drill and sheetrock screws

Figure out your channel location on the wall and then screw the 1x6 from one end of the cut to the other. Use screws liberally. Bonus points for using a level on the board. Mark the upward orientation on the board.
Set the depth on your rotozip to just barely cut all the way through the rock and then add an extra 1/16" for those areas with a bit more mud. You want a single, clean cut from one pass. Attach the shop-vac to pull most of the dust out of the tool, and using the board as a guide, make one pass all the way around. This will give you a clean, straight line with about 8" of working space to drill through the framing underneath. Leave the vac running as you power off the rotozip, and vacuum out your entire cut line. Clean as much dust as possible at this point.
Now, remove the board WITH the sheetrock attached to the back of it. You may have to pull a screw or two through. This will keep the long, thin piece from breaking, or coming apart at the seams. Set the board and sheetrock aside. Remove any screws that protrude from the studs.
Drill the studs, run your wires. Using a pencil, mark the stud locations on the sheetrock above and below the cut(or the sides on a ceiling).
Re-install the sheetrock that you removed, using the marked stud locations. You should have room around the edge of the board to cleanly screw down the removed piece where the screws will be covered by tape. Remove the board from the sheetrock and tape/texture away.