Step One: I laid the mattress over the fabric, left the full width and cut the top and bottom to match. Then I pinned the corners together and sewed. We went with a foam mattress, which is super light, and made this part of the project very easy.
Step Two: Once I had the fabric squared up on the mattress, I did a zig-zag stitch around the raw edges. I do not have a surger. I did this in lieu of folding a hem because the fabric is only 42" wide. I needed to maintain all the width possible or I would run the risk of it being too short on the sides.
The skirt was even easier. It still needs some tweaking. I will come back to that later. But the moral of the post: do not spend a billion dollars on cute crib sheets! If you can turn on your sewing machine, you can make your own crib sheets! And with all the fun fabric out there, the possibilities are endless!
Another Bonus: I like it so much, I'm no longer sad about going sans bumper.