For the front of the duvet cover, cut a piece of fabric to 36 by 50 inches, which is 1 inch wider and 2 inches longer than the comforter. To make a 1-inch machine hem across the top, turn the raw edge under 1/2 inch, and press; turn under again 1 inch, and press. Machine-stitch close to the turned edge.
Cut two 52-inch lengths of twill tape, and starting at the bottom, pin one length of twill tape vertically up the front, about 11 inches from the side. Using matching thread, sew the tape very close to both edges, leaving 4 inches at the top. Sew across the width of the twill tape at this point to secure it. Do not cut the excess twill tape; it will be used as tie closures. Repeat on other side.
For the back of the duvet, cut the width of a flannel crib sheet to 36 inches. To make a 4-inch flap on the open end of the cover, fold the finished top hem of the sheet back so that the wrong side of the hem shows, and press. Pin front and back of the duvet cover wrong sides together with the back's edges showing over the front by about 1/8 inch. This will allow some give for the flap to fold over. Sew around three sides, leaving a 1/2-inch seam allowance and the top open. Trim seams. Sew diagonally across the corners, about 1/2 inch in; trim corners.
Turn cover inside out, and press. Sew second seam, leaving 1/4-inch seam allowance and the top open. Turn right side out, shape corners, and press.
Fold the end of a 9-inch length of twill tape under 1/4 inch, and press; repeat with second piece of twill tape. Turn the flap over the front of the duvet like an envelope, and sew both pieces of twill tape to the underside edge of the flap, lining each up with the set of twill tape running down the front of the duvet cover.
Hand-sew strips of twill tape to the corners of the comforter and the inside corners of the duvet cover. Tie comforter corners to duvet corners to keep the duvet from shifting. Once the comforter is inside the duvet cover, turn the flap over, and tie the twill-tape closures into a bow.
Soft, washable cotton fabric
Twill tape, and matching thread
Flannel crib sheet
If you don't want to limit yourself to store-bought coordinating crib sets, a handmade duvet cover is a wonderful alternative. The front of this comforter is made from soft, washable cotton, and the back is an even softer flannel crib sheet. Be sure to wash the fabric before cutting to allow for shrinkage.
It's important to note that the American Academy of Pediatrics recommends that parents not use comforters or soft bedding in cribs except for decoration if the baby is less than 12 months old.
This duvet cover will fit a 35-by-48-inch comforter.