STEPS FOR MAKING A RUFFLED DUVET COVER
I first measured the duvet insert and bought enough fabric to cover both sides plus a generous overhang for the envelope closure section.
I opted for an envelop closure type duvet cover, so at the top I left approximately 10 inches of overhang.
I cut this overhang off, leaving roughly 2 inches for seam allowance. Before stitching the sides closed, I folded over & pressed the top edge of the main long top piece.
With both pieces hemmed and laid out, I also found I needed to trim some additional fabric off the bottom to give me a consistent seam allowance around all the edges.
I removed the duvet insert and laid the bottom piece down, with the rough side of the envelope closure lined up on the top edges, right sides together, and pinned in place.
Then I laid the top piece over it, right sides together, but ensuring the topped hemmed edge was a few inches down below my pinned edges.