To make a 16-inch-square pillow, for example, cut two 17-inch squares of fabric. Pin them together, right sides facing, with raw edges aligned. Sew the fabric together along three sides, 1/2 inch from the edges. Turn right side out, and press. Insert a 17-inch pillow (using a pillow insert that's slightly larger than the case gives a plump, overstuffed appearance). Slip-stitch the fourth side closed.
To make a pillowcase with a buttoned opening in back, you'll need one 17-inch-square piece for the front and two overlapping pieces for the back, each 11 3/4 inches by 17 inches.
Double-fold along one long side of each of the back pieces, making a 1/4-inch-wide fold and then a 2-inch-wide fold; press flat. Stitch along the fold lines. Lay the two back pieces right side up, so their hemmed edges overlap by 2 inches; pin to hold in place.
On both pieces, use tailor's chalk to make marks 6 1/2 inches from the top and bottom of the hems; the marks should be 4 inches apart and between 1/2 inch and 1 inch from the edge of the top piece, depending on the size of the buttons. Sew buttonholes in the top piece and buttons on the bottom. Button the pieces together.
Pin the buttoned back panels to the front, right sides facing and raw edges aligned. Sew around the perimeter of the pillow. Trim corners, and press seams. Turn case right side out, and insert a 17-inch-square pillow.
For a closed pillow, begin with two pieces of fabric cut to identical dimensions -- the size of the final pillow cover plus 1 inch for the seam allowance.