I started off by cutting off the seams. Then, I cut out the 10.5" squares from each corner (this much, I DID get from my online research). I stitched those up to make it "fitted" and then found some extra elastic that I had here. I cut that up into 8 equal pieces (I was planning on making 2 mock-ups before cutting into the real thing). I stretched them out and stitched them on. VIOLA......it was too big for the mattress. So after a few quick recalculations, I went back to the drawing board and started over on another top sheet. I finished that on up and.....IT FIT PERFECTLY!!!!!!!!!!! So then I followed the same steps again:
Cut material to 58" by 92".
Cut 10.5 inch squares off each corner
Stitch them to make them "fitted"
Sew in 12" of elastic in each corner to help it stay on the mattress**
This is the end result:
**NOTES: Since I used PUL fabric, I didn't make a hem or a casing for the elastic....it doesn't fray. If you're turning a top sheet into a fitted, you'll want to at least surge around the edges if you don't want to hem it up or make a casing.