Sitting on the floor

There's a section on this at the end of the stacksitting chapter in the 8 steps book.

If your knees are healthy, it works well to sit the way the Japanese do when they eat. If your ankles complain, put a small roll under the ankles. If your knees complain, sit on a firm cushion like a Zafu placed sideways, so the pressure is off your knees.

If you use one arm to lengthen your spine a little, it works well to modify the Japanese sitting position and shift your bottom over to one side with both legs bent to the other.

If you use a cushion (e.g. a Zafu) on the floor, it will probably work for you to sit cross-legged on the edge of the cushion. The less acute the angle at your hip joint is capable of (whether because of muscle inflexibility or bone structure), the thicker your cushion needs to be. You may do best with a little bench (like are sold in meditation supply stores) or a little stool.

I don't recommend for people who grew up using chairs, raised latrines and generally not living on the floor, to take up sitting cross-legged on the floor in an unaided way as adults. The arguments are here:  http://gokhalewellness.com/forumtml