Wash and dry the mattress protector. This allows for any shrinking, if any, to occur before measuring and cutting.
Measure, mark, and cut the fabric to get the desired dimensions for the two pads.
Fold both pieces in half to line up the curved corners with the new corners. Trim the new corners to match the curve of the original corners.
If you do not have a serger, you could use an overlock stitch or zigzag plus a straight stitch on your sewing machine.
Test the stitch settings on a scrap piece of fabric to match the original serged edges.
Since there is no extra fabric on this project, I used two pieces of drop cloth to roughly mimic the thickness of the mattress protector while testing the stitches.
Disable the knife of the serger and leave about two inch chains before and after serging.
Serge the new edges. Include a 1-2 inch overlap on each end to secure the cut stitches of the original serged edges.
Weave the thread chain through the stitches on the backside with the tapestry needle to finish.