Fabric: A sturdy material such as a cotton/polyester blend. Try 100% cotton duck fabric. It is important to find a fabric that you will not slip off of when you are meditating. See coment below under 'pieces'.
Stuffing: Kapok is the more traditional fill. Buckwheat hulls are a common alternative and work great. Try alternatives; foam, cotton waste, mixtures. Check a few in stores or borrow a friends to try out. You can always pull the stuffing out and refill. It is your bottom so make it happy :)

a) Length of cloth 59 inches x 6 to 9 inches wide. A wider strip will make the cushion higher.
b) Two circles of cloth. A diameter of 11 to 13 inches - depending on how large around you would like your cushion to be.)
       More experienced sewers could try panels (of different cloth?) for the long strip a and concentric circles, patchwork or quilting for the two circular pieces b. You, or someone, will be spending a lot of time with your cushion so investing time is worth it in the long run. Don't be 'bullied' into the standard "serious 'Zen' meditators black-on-black". Likewise, don't get hung-up fussing and deliberating. It is essential a 'tool'.

Pleat the length of cloth (piece a). There should be fourteen 3/4 inch pleats, 3 inches apart.
To pleat:
a) Beginning 6-1/2 inches from the left edge of the length, make three marks, 3/4 inch apart, thus marking out the first pleat:

Three inches after the first set of pleat markings, make the second set, as indicated above. Continue doing this till you have 14 pleats. When you finish, the last pleat marking should be 3 inches from the right edge. (If you wish to have narrower pleats simply increase the number of pleats.)

b) Next, iron the pleats and pin them. They should all be folded and ironed
in toward the left-hand side. For each set of pleat markings, fold the third in toward the first as shown, and then pin as shown at below:

Now, having completed the first step, take the right edge of the pleated strip cloth and pin it to the left end of the strip, 3-1/4 inches from the left edge. YOu may find it easier to stitch the ends of the pleated cloth strip together on the top and bottom, while leaving a hole in the middle to stuff the zafu once it is sewn together.)

Mark each circle of cloth at four equidistant points. Turn pleated length of cloth inside out. Pin each circle to the pleated strip, one circle to the top edge and one to the bottom edge, at each of the four points.

Next, ease (pin) all the pleats in to the circles, top and bottom. Sew the circles to the length of cloth.

Turn inside out and stuff with kapok or buckwheat hulls (through opening in the side that the zafu will have).

Your zafu may end up with a bad-looking hand stitched opening on the side. To cover it up try sewing an extra piece of fabric on for a handle. Take a (any colour or texture) piece of cloth 2-3 inches wide and little longer than the zafu is high. Sew the long edges together and then turn it inside out. Iron with the seam in the middle. You could try a decorative stitch down both edges or embroider your name (if it will be on the floor with many other zafus). Sew the handle over the opening into the top and bottom seam line as you stich the top, bottom, and sides together.

Most important of all.....
sit on it regularly!
Love your handiwork by regularly meditating :)