Several ways to do it. You can make a bench run cornice you make then stick up, or run one in place with a jig you make... I did one a few months ago I'll post some pics.
Had to reshape the corner. A miter rod would be helpful here but I didn't have one so used a very straight trowel.
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This is the jig I made and I traced the profile onto a piece of 16 gage metal and cut it out. I ran everything in place since the ceiling was all over the place I made a track on the wall and ran it to that and then just filled any areas above the cornice after.
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Here is a longer section patched after the a wall was removed. Somebody else tried their hand at it before I got there which made things a little more difficult but it worked out in the end.
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