Cut a piece of fabric the length and width of your crib mattress + thickness + 4"
If your mattress is 27”wide, 51” long and 6 inches thick.
Then you would have 27+6+4= 37" wide
51+6+4 = 61" long
Then cut out the corners like below:
Fold wrong side out, line up corners and sew along straight edge.
Fold edge over and sew hem / pocket for elastic.
Insert elastic into pocket all the way around and connect.