grann of 6

If you want it to look like a quilt I would put some thin batting in it between the quilting and the muslin backing. You can use a sheet for the backing too. If there will be a comforter in the duvet you won't need much. And if you have a serger that is a good way to finish the seams; makes it very washable. Once you have made the front, then finish the rest of your duvet as with any duvet directions. You can use any size seam you want, just allow for this in your measurements. I usually make mine 1/2" seams, then when I am calculating measurements all I have to do is add an inch to the measurement. Not so much room for error that way.