Measure the mattress length, width, and depth
Add extra fabric for seam allowance and pocket depth
Wash, dry, and press the fabric before cutting
Cut one large rectangle for the sheet top
Cut four corner squares equal to the mattress depth plus seam allowance
Fold the fabric with right sides together
Sew the corner seams to form the fitted corners
Finish the raw edges with a serger, zigzag stitch, or French seams
Fold and hem the outer edges of the sheet
Create a casing around the elastic edge
Thread elastic through the casing
Stretch the elastic evenly and sew the ends securely
Close the opening in the casing
Test the sheet on the mattress and adjust if needed