I've been meaning to make a new cover for my feather comforter for some
time now. Since the cover is fairly large, and since it will be filled
with something fluffy, this is a project that is very forgiving of
stitching (and even cutting) that isn't straight.
We have a queen sized bed and I wanted to make some matching pillowcases and throw pillows, so I bought a king flat sheet and a coordinating king sheet set to use for fabric (bought after the new year, when linens are on sale, this cost me less than $25). I think this was plenty for us but if you have a king bed you might need to buy another flat sheet or some fabric.
Before you ever get started, make sure to pre-wash and dry your fabric so that it has already shrunk as much as it is going to. Iron the fabric so that you make accurate measurements and cuts.
To make the duvet cover, you are really just sewing large rectangle that has an opening in one end. Measure your comforter width and length and measure, mark, and cut your two flat sheets so that you allow for a seam allowance and, if you choose to do this part, the french seam/sham flange edge. I generally do a 1/2" seam allowance and I did about a 1" flange so I added 3" to my comforter dimensions. I would not recommend making the duvet cover oversized for the comforter because I think you tend to get more lumping that way.