This isn’t very complicated. Perhaps look up on YouTube how to patch drywall? As others have said, you definitely need blocking across each edge. If the drywall is crumbling, as you said, why? Did it get wet? If it did, you may need to cut out the section that has lost its ability to hold a screw. If it’s the whole sheet, so be it. They’re like $15-$20 each. Not worth breaking your head over IMO