Position
You can lie down if you wish, but I find I'd be more likely to fall asleep that way. If you do fall asleep, by the way, it's no bad thing.
Sitting down on a chair with a back but without arms became my preferred method but it's entirely up to the individual.
Sit up, with your back straight, and hold your head and spine in alignment. Shoulders back. Rest you hands comfortably on your lap, interlocked and cupped, with palms turned upwards. Thighs should be parallel to the floor. Back can be supported by the chair but the head should not.
Jaw should be relaxed with the upper and lower teeth not touching but mouth should be closed. The tongue should rest comfortably in the mouth with the tip of the tongue touching the roof of the mouth just behind the teeth.
Close your eyes and try to empty your mind. When you begin, you will find it difficult not to think. You may find that thoughts keep intruding. When this happens, don't try to force the thoughts out. Observe them impartially, and then let them slip away.
Observe the tension lifting away from your body. Some people like to go upwards through the body from the feet. You may like to go downwards from the head.
Either way or if you prefer to just relax, you will find your appreciation of this existance – just sitting and breathing – very pleasant and relaxing.
Don't worry if you hear little noises around you. Don't cut out all the forces around you but to become one with them.
There are many methods of doing this. Once again, please explore the links section to find your preferred method.