Good instructions for making your own zafu (meditation cushion) can be found here. Personally I find that 12″ diameter top and bottom works well combined with a 6″ x 58″ band in between.
Here is a demonstration of my own recent zafu making following the above instructions. You can stitch by hand or use a sewing machine.
First cut the top and bottom circles of fabric.
Next cut the band that will join the top and bottom together and pin pleats as described (mark 6.5″ then three more lines at 7.25″, 8″ and 8.75″ respectively from the last pleat).
Evenly pin the top and bottom to the mid-band and sew together.
Turn right way out.
Gather a bunch of old clothing or material for stuffing your cushion . You can also buy kapok to stuff with.
Stuff the zafu, making sure the stuffing goes evenly into all parts of the cushion.
Fold the last bits of fabric over and sew tight.
To add a nice finish, you can add a strap of material and even a wee bit of embroidery.
Et voilà, it is time to sit!