I realized we were short on fitted sheets too (it was quite a night), so I went to Home Goods and for $24.99 bought a Ralph Lauren twin sheet set that complimented one of the flat sheets we already had. I used one old and one new flat sheet to make the duvet cover - all for under $30 bucks and a little labor!
Here is what I did, ironically, it is very close to the "DIY Decorative Pillows" I recently made.
First iron the sheets - admittedly the most tedious part of the entire process. No photo necessary here!!
Then fold-up one end of both the sheets about 3 inches, so that you have a nice place to hide the buttons to close the duvet cover.
Put the right sides of the fabric together and pin. I laid out all of the fabric with a duvet to double
check the size and seam allowance.
Sew 3 sides of the sheets EXCEPT the bottom where you are going to put the buttons.
Flip the duvet cover right side out and press the seams.
Mark off the button and button hole placement. I like to space them about every 6" - 8".
Add your button holes (most sewing machines have a "Button Hole" stitch) and sew on your buttons.