Are you wanting to make a duvet cover? I have made them in the past and used sheets that are a size larger than the blanket I was covering. Example: Queen size blanket, so I bough king-size sheets.
I cut to size - about 2 inches larger than the blanket I was covering, then sewed three sides. I sewed hems on the open sides and made buttonholes very 6-8 inches to close.
Not fancy, but functional.
I hope this helps you. :)