I don't know many people who use these, but I have been thinking about using one lately. This may be a coincidence, but I was painting yesterday, and I accidentally (I should have moved it) got paint on a comforter I had in the closet. By finding this tutorial, it saved me from DI-ing it or chucking the comforter.
1. lay 2 sheets of the same size facing each other.
2. lay an existing duvet cover/comforter of desired size on the sheets.
3. leave a .5" margin and mark a cutting line around 2 sides of the sheets (leave the top and one side uncut).
4. Pin all sides of sheets together and sew around 3 sides leaving the top (or bottom) uncut side open.
5. Fold the sewn sheets in half length wise so you can find the middle and mark it, open the fold and measure and mark 12" from the middle to each side and sew from the sides to the marking (see above).
6. Ta Da!!! your done, congratulations!
Tutorial found HERE.
Check THIS post out for another way to make a duvet cover: