I've written extensively about my history of and preference for duvet sleeping here (my goodness I can natter on about the most trivial things). So, when E and Z got down comforters for Christmas from their grandfather—white down comforters—I knew they needed duvets to keep those comforters clean, stat. When I was in Target one day last week, I looked at their selection, but they all came as sets, which included such essential items as shams and dust ruffles, and cost $80-$90!!! So I took myself to the sheet aisle and picked up four flat sheets for a total of $44.
I ran the sheets through the wash, pinned them together, right-sides facing (I did not even bother to iron—shh!), and sewed all the way around three sides plus about 1/3 of the way in from each of one short side, leaving a small opening for inserting and removing the comforter, but also hopefully, keeping said comforter from wiggling out of the duvet (when I made my duvet, I put buttons across the bottom, but they didn't work out as intended; I hope the partially closed bottom is a better solution.
The color selection of flat sheets at Target was a little uninspiring—lots of neutrals. For Z, who has a preference for stark, Scandinavian, black-and-white decor, I used gray on one side and coral (which I say is much more red than pink, don't you agree?) on the other, and for E, I used light blue and beige (told you the color selection was dullsville). Of course I got it wrong, and E wants the red and gray. I just can't win, but at least the comforters will stay white.