Unfold the duvet cover, lay it flat, and smooth out any wrinkles. Fold the duvet cover lengthwise into an accordion—depending on the size of the duvet, the number of the folds will vary. For the queen-size duvet used here, the accordion of fabric was 6 folds high and about 10 inches wide. After the duvet has been folded into a long strip, fold it into triangles like the pillowcases. Secure the corners of the triangle with tightly wound rubber bands, placing them about 4 inches in from the end of the triangle. After all the bedding is securely bound, soak it in the prepared soda ash solution for at least 1 hour, until the cotton is fully saturated.