Measure from the top of the mattress to the spot where you want the daybed cover to stop. On most daybeds this will be about 1 inch above the floor. This measurement will be the size of the cover corners.
Lay the comforter on a flat surface with the longer edges at the top and bottom. Measure up the right edge from the bottom up and make a mark at the corner measurement amount. Measure up the left side the same distance and make a mark on the edge there.
Measure the bottom edge from the right corner in toward the center. Make a mark at the corner distance. Do the same thing on the bottom edge starting at the left corner. You now have a comforter with two marks on the bottom edge, one on the right edge and one on the left.
Mark a vertical line from the mark on the bottom right edge up toward the top of the comforter. Mark a horizontal line from the mark on the right edge in toward the center. Mark both lines until they meet. Do the same thing with the two marks on the left side of the comforter. The comforter now has a square outlined on the two bottom corners.
Cut the two corners off the comforter, cutting along the marked lines.
Pin bias tape or seam binding along all four raw edges from the two cut-out squares. Stitch the bias tape to the edges to create a smooth edge. You now have a comforter with one long flap and two side flaps.
Lay the comforter on the daybed with the longer flap in front. Position the cover so the corners between the flaps sit right on the corners of the daybed mattress. Tuck the two side flaps between the daybed mattress and frame and the excess comforter fabric behind the mattress.