If you are very tall, with long legs, make sure that your hips are higher than your knees, either on a chair or on a cushion.
If you choose to sit on a chair, pick one that has a flat seat that doesn't tilt too much toward the back. If you are short, like me, you will want to put something on the floor for your feet to rest on, taking a little bit of weight. You don't want your legs dangling uncomfortably. If you are very tall, with long legs, make sure that your hips are higher than your knees, either on a chair or on a cushion. If you don't do that your back will start to hurt pretty quickly.