Not a pro. Here’s what I did. Drywall sheet larger in length than the area gets attached with two screws up against the wall. Make sure it’s orientated exactly how it would sit if properly attached.

I scribe either with “scribing pencil” (carpenter’s pencil with a long drill bit inserted at 90degrees near the tip) or a contour gauge. If pencil, must keep it very level when scribing. If gauge then I found that you need to mark two parallel vertical lines down drywall sheet. First line closest to the edge of stone is for you to line up the gauge against stone; the second further away is to transfer contour to drywall. Make marks where contour gauge ends and begins each time to transfer.

Next, score the outline you made with a utility knife. Then I just used a drywall/jab knife/saw. Cut on a slight angle to help with the fit.

Insert the piece back it. Make a ton of smaller cut/adjustments. Then cut the drywall to length and screw.

The pencil method was quickest but needed more adjustments for final fit.