What you need:
- Fabric (I used flannel)
- Sewing Machine
- Iron and ironing board
To find out how much fabric you need measure the mattress and the width of the mattress. Since this mattress was only two inches wide I took the length+width and added 6 more inches for the seems and to allow for the sheet to tuck under. I did the same thing for the height.
Then you cut out a square from each corner. I cut 7x7 inch squares out so that it left a little breathing room for the corner seems. If I cut 8x8 inch squares (the width + the 6 inches I added in) I might not be able to stretch it over the mattress when I finished.
Next take the two edges (marked below with arrows) and put them together so that the right sides of the fabric are touching and pin.
Do this to all four corners.
Take it to your sewing machine and sew each one. Then press the seems open.
Now fold the bottom edge over 1/4 inch and press along the entire sheet.
Fold again and repeat. Since my elastic is 5/8 of an inch I folded my fabric over a little more than an inch this time.
Take it to your sewing machine and hem all along the edge leaving an few inch gap to thread your elastic through. I always pin the end of my elastic because it makes it easier to push it through. The size of your elastic will again depend on the size of your mattress. I took my length+length+height+height and added a few inches, In the end I had enough left over that I could gauge where I wanted to cut off the excess.
Once your finished threading it though put the two ends of the elastic together with right sides touching and sew them together. Tuck them in and sew up your hole.
Now you are finished. The large piece of fabric you started with should look something like what you see below. Time to stretch it across the mattress and put that baby to sleep!