There are several different kinds of sitting styles for Zen meditation. You should choose the one best suited to you depending on your legs and body structure. The postures are: full lotus, half lotus, Burmese style, small wooden bench and backless chair. No matter which posture you use, there are important points to practice, which are the same for all sitting styles:
(1) The spine is kept straight in a 90 degree angle to the floor.
(2) The head is held so that the nose is in a straight line with the navel.
(3) The chin is tucked in slightly.
(4) The ears are in line with the shoulders.
(5) The arms are held slightly loose at the side.
(6) The hands are held below the navel, in front of the Dan Tien [pronounced: dahn tee-en], which is about two inches straight down from the navel.
(7) The hands are held with the left hand on the right palm and the thumbs joined together in a circle. This is the cosmic mudra symbolizing the universe and the perfection of inter-being.
(8) The rear Dan Tien is always pressed towards the front in order to prevent bending of the spine.
(9) The eyes are half open, neither staring intently (which may causes headaches), nor completely shut (which can lead to daydreaming and dozing off). The angle of vision is approximately 45 degrees in front of you, towards the floor.
(10) The mouth is closed so that all breathing takes place through the nose. The teeth join naturally as a result of the chin being held in.
(11) The tongue is held against the roof of the mouth to prevent saliva from filling up the mouth.
(12) The position of your body on the cushion is very important. Choose a cushion (zafu) that fits your body size, i.e. it should be neither too high nor too low. Sit more towards the front of the cushion so that your legs and knees naturally rest on the meditation mat (zabuton).
It is difficult to maintain concentration with an overly relaxed posture. Posture which is too straight or too relaxed will likely produce physical problems, such as pain in the back or in the legs and shoulders. The ideal posture is neither too straight nor too relaxed. It is soft and gentle and outwardly reflects a compassionate mind.
It is best to have an experienced person check your posture frequently in the beginning until you establish a good solid physical foundation for sitting. As long as you have correct posture some pain in the legs is not a problem and you shouldn't worry about it. Only if you are sitting incorrectly can it become troublesome.
2 - For a visual demonstration, please open the file below or watch the video (both are just a few minutes long):