How do you make a duvet cover

I think you are on the right track. Sew three sides of the sheets to make an envelope. Insert the duvet. It helps to have two people for this. To make the duvet and cover permanent, sew the open end with a blind hem stitch. To keep them separate for washing, you have several choices to close the end, which is usually at the bottom of the bed.

1. Sew in a very long, strong zipper made for this. It doesn’t have to go completely across the bottom because the comforter inside can be folded and stuffed. Then you shake the package to get it smoothed out as much as you can

2. Sew in Velcro in 12” strips with 6-12” gaps.

3. Turn the edges under and hem them all around the end, leaving the envelope open. Then, on the hem, on the top half, sew very large buttonholes. On the inside of the bottom half, sew on very large buttons.

4. Sew on the largest loops and hooks you can find. The bra extenders for wide-back bras work great. A sewing and fabric center should have them. I would sew one set every foot or sew, after hemming the edges.

You will probably find other solutions, but perhaps these will get you started. Best wishes ☺️

duvet cover as in feathered filled quilted blanket? 2 flat sheets should do it. Center the duvet on the sheet, find your seam allowance, remove duvet sew top to bottom leaving a 12 inch gap in bottom of the case to stuff the duvet in. Use sew on snaps to close the opening.

Run Velcro along if you don't want to use poppers or buttons and buttonholes.

You could simply make ties for the opening, or buttonholes and buttons, or buy one of the zippers by the foot (although I haven’t had success with those) you could also sew Velcro on the opening

I have two duvet covers. Both use buttons for the opening.

I am using mine as a quilt. It was to hard to stuff a quilt into it. Plus is never played flat inside the duvet cover. It is made of heavy material too. It's kept me warm this winter. You should be able to see the buttons too.