You need the rock to have a good tight fit in the corners. Inside corners must use paper tape, not the fiberous one.
You might need the inside corner tool also. I would then fill the area with mudd, then apply the tape then more mud then apply the tool for inside angles.
Let it dry and then sand and apply more due to shrinkage.
You could also use the inside metal edging for both inside and outside corners. Looks sharp and works well.
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