I did it on all of my king sheets when I changed to a queen bed. I made the seam right where it went over the side of the bed. That way you never had to lay on the seam. I just put it on the bed inside out, pulled tight, then folded it to fit and marked with pins. I stitched it on the machine.
I also had an electric mattress pad. For that I just folded the excess under the mattress and pinned it to itself.
It's been about 10 years and all of these are still working fine. I hang my sheets on the line, and they last forever. Even in the winter there are warm days when you can hang them out.
I had a valance which was a king size, my bed was a double, all I done was sew a seam straight up the middle, after I had measured it, then cut off the excess, worked like a charm.
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