This is where you need to look to see what I am doing. Place the short cut edge of one corner right sides together and sew. Do this to all for corners. Now the sheet is fitted. Now I would serge around the whole edge or you could do the double rolled hem if you don't have a serger.
Next thing is to put elastic around each corner.
Look at your sheets at home to see how they did it. The sheet is finished. I like my sheets to come well under the mattress. Since they show it displayed on Samantha's bed on the pattern jacket it's probably a pretty good fit. If you want to tuck a pillow under, a great way to reduce bulk is to have the top section of quilt turned in the opposite direction.