Twin beds in my PC … What a pain to make and nothing standard you can buy fits plus they are different sizes. This is what I did and it's working fantastic.
I went to WalMart and bought sheets. $4.00 a piece ( 2 full and 2 twin/single ) with a low thread count, because I didn't want to practice with expensive sheets.
Throw them in the dryer to get out most of the wrinkles first. Remember you are going to be working inside out on the fabric. Fold the full sheet lengthwise and sew together the long side and one end, sew the seam a ¼ to 1/2 inch from the edges. Measure the depth of your mattress – that will be the depth for your fitted corners … on the sewn end, sew 2 box corners carefully cutting off the excess material "after" the corners are sewed.
Take the mattress off the bed and slide it into the "mattress case/fitted bottom sheet" adjusting your corners to fit where they should…this was actually quite easy to do.
I was going to put a drawstring at the open end but it bunched up too much.
I folded the open end over like wrapping a gift and used 2 or 3 large safety pins to close the end snuggly. If I come up with something better I will share it.
Make the bed with the top single sheet, tucking the excess under the mattress on the wall side and foot of the bed. I probably could make the sheet narrower and hem the side but I am comfortable the way it is for now.
I liked it so well that I made mattress covers the same way and put a 2 inch mattress topper inside on the mattress, "WHILE" I was slipping the cover over … WOW
now that was a chore ( you want the topper to fit flat and stay without folding or creases ) and luckily I had a friend to help. The extra pair of hands for doing that was surely appreciated. As far as making the mattress covers …. depending on how thick your mattress is and how thick the mattress topper is, you may want to buy a queen sheet and measure to fit for thicker covers/mattress case/bottom fitted sheet. If you are putting a mattress topper in your mattress cover (not bottom fitted sheet cover) ... have it snug enough as to not shift around while you are sleeping.
I also bought the matching pillowcases. The WalMart sheets are not perfectly squared so don't stress yourself thinking you did something wrong. I found this was a break two nails and a 4 cups of coffee project.
With this being said I have bought better sheets, higher thread count, and made 2 sets of sheets. All is working well