The standard measurements of a crib mattress are 27"w x 52"l x 4"d
Cut a rectangle of Interlock, Jersey, Ribbed Knit, or Tricot 39" x 64" (Note: If using an interlock or ribbed knit and the fabric is at least 60" wide you will only need 1 1/8 yd, other wise you will need 1 3/4 yd.)
Cut a 6" square out of each corner of the rectangle
Miter each corner with an overlock stitch or serger
Using 90" (2 1/2 yd.) 3/8" Swimwear elastic serge or zig-zag to the wrong side of the fabric. Be sure to overlap the ends approximately 1/2". If desired, you can then turn the elastic to the wrong side of the sheet and zig-zag at the inside edge to give a more finished appearance.
Note: If your mattress is a different size you can do the following to figure the amount of fabric needed
The width of the mattress + two times the depth + 2" for the width of the fabric. The length of the mattress two times the depth = 2" for the length of of fabric.
Example: Mattress is 28"w x 54"l x 7"d. You will need a piece of fabric 44"x70"
To figure the amount of elastic needed, add the width + length then multiply by 2 the multiply that by 60%. The resulting number will be the amount of elastic needed.
Example: for the above mattress you will need 98" of elastic
Special Note: Please see How to make a baby crib sheet if you want to use a woven fabric