I bought 3 flat twin sheets and one king flat sheet. On the twin sheets I cut off the top 3 inches and the hems leaving just fabric. My queen comforter was 92"x92" so that is what I based all my measurements off of. If you have a different sized comforter, you may have to adjust the measurements a little. I ironed the first twin sheet and laid it out. I cut it to measure 93" tall by 62" wide. Then with a fabric marker (one that will come out when washed) I marked 3 vertical lines from top to bottom all 15 1/2" apart. Like this:
Then with the next sheet I measured out and cut a piece of fabric to measure 93"tall by 31" wide. I then marked a vertical line down the middle. Then sew these two pieces right sides together, leaving you a piece of fabric that is 93"x93". Lay aside the extra fabric from the second sheet and get the third sheet out. Cut a piece that is 66" tall by 93" wide. Match up the 93" wide sheet you just cut with the bottom of the original piece. Then sew right sides together. Once these are sewn together, continue the vertical line all the way down the newly added section. Making sure the lines are 15 1/2" apart. Now it is time to gather/ruffle the fabric. If you have a ruffler foot, I did the middle setting. If you don't, just baste stitch on the lines you drew earlier, all the way down and then gather your fabric up until in measures 93".
After you have all your fabric gathered, it is time to make the ruffles. From the fabric left over from the 2 twin sheets, cut strips measuring 4" wide by 155" long. You will have to sew some pieces together to get it that long, but in the end you need 5 strips of 4"x155". After they are cut, iron in half and sew shut. Leave one end open, sewing the other shut.
Then turn inside out. The easiest way I have found to do this is to place the sewn end on a dowel rod and then gather the fabric until it is turned right side out.
After it is right side out, iron the fabric so the seam is on the bottom. Then ruffle the fabric using the same method as above. Once it is ruffled, lay it on the vertical lines you drew and sew in place.
Now you have the top piece done. Next you need to make the back. Take the king sheet and cut two pieces. Cut the first piece to measure 93" wide by 32" tall. Then cut the next piece to measure 93" wide by 62 1/2" tall. For mine I cut it so that the hemmed part of the sheet would be the opening of the duvet cover. (That way it saved me from hemming.) If you would rather hem the opening yourself, add 1" onto the length. When you are done line up the 93" hemmed pieces right sides together. The 93" wide by 32" tall piece needs to be 1 1/2" down from the top of the 93"x62 1/2" piece. Then sew 10 inches in on each side. You can then choose to add button closures, I just left mine open. Then lay the top part and the bottom part right sides together and sew all the way around the four sides. Turn right side out and insert your comforter! Enjoy!!