I measure the cushion, and then draw the outline of it's shape on the fabric using taylor's chalk (regular chalk work too, but not as well), adding a 1/2 inch seam allowance all the way around. Cut 2 of these, remembering to cut one on the right side of the fabric, and one on the wrong side. For the cushion band, determine on what side you want the opening.
Cut one piece of fabric the length of the other three sides plus 1 inch for seam allowance. The width should be the height if the cushion plus 1 inch. I've never done Velcro, but here's how I'd do the opening. Cut two pieces the length of the opening end of the cushion plus 1 inch, and the same width as the other strip. Turn over 1/2 inch on each long end and hem.
Overlap the short ends of these pieces wrong side to right side and stitch the short ends together so you have one piece with the hemmed over ends overlapping. Fold this in half short ends together and cut a small notch on each long side to mark where the center of the long side is. Take the other long strip and mark the center of each long edge the same way. Stitch the strip, wrong sides together to the short ends of the other long strip to make a big circle (note, do NOT make a mobius strip). Take the big top and bottom pieces and mark the centers of the edge you want the opening on, and the side opposite that (these instructions are only for cushions that are basically square or rectangular, triangular cushions require more precise measuring). Match the notch on the band (the big circle) on the opening side to the notch on the large piece, wrong sides together. Stitch, clipping any curves and pivoting the fabric and clipping the corners. The notches in the other side should match up, you can stretch the band or large piece a little to make it work. Continue around to the beginning, and then match and stitch the other flat piece to the band, wring sides together. Stitch Velcro as needed at opening, or apply snap fasteners. One could do buttons too, but making the button holes before stitching the band to the top and bottom would be easier than after.Turn right side out, and stuff the foam in. If you wrap the foam in something called "Slip'Eze", a very thin Tyvek, it will slide in more easily. I'll do the rundown on zippers and cording if anyone is interested. I can generally do a simple square cover, with cording and a zipper, in under an hour if I don't have to match plaid and the old Singer cooperates.

I can demonstrate much more easily than I can describe. Lessons a