We're going to start with the easier postures first. One thing I often see in beginning meditators is a desire to contort themselves into a posture that is too hard on them. This results in discomfort, distracted meditations, and even physical damage. Be kind to yourself.
You can meditate perfectly well in an ordinary dining-room or office chair. The only thing you have to do to modify the chair is to raise its back legs by maybe an inch or so (2.0 to 2.5cm). This allows you to sit upright without having to either hold your back rigidly, or leaning against the back of the chair. Blocks of wood, or even telephone directories, can be used for this.
The meditator in this photograph probably needs to raise the back legs of his chair another half-inch or so, so that he can sit more upright.
When I use a chair to meditate I like to have only the very base of my spine touching the back of the chair. It's best not to lean back in the chair - I think it encourages underachievement! Rest your hands on your thighs, palms down. Have your feet flat on the floor if you can. If your legs are very long or very short compared to the chair, then this might not be possible. If your feet don't reach the floor, then you can use another phone book to rest your feet on. If your legs are too long, then ideally you should find another chair.
Some office chairs are perfect for meditating! Set the seat so that it is slightly tilted forward, and make sure that the backrest is only making very slight contact with your lower back. Adjust the height so that your feet are flat on the floor.
