How to meditate Zazen is not a meditation technique, it is the dharma gate of joyful ease.
Eihei Dogen

The quote above is how Dogen, a thirteenth century zen teacher, described shikantaza, or "just sitting". What Dogen is pointing at is that Zazen is a complete experience of being for its own sake; it is not a means to an end. An interesting consequence of this is that you can't do zazen right or wrong. All you're going to do is sit, and experience whatever is going on. This means feeling whatever you feel (emotionally or physically), think whatever you think - and just watch. There will be parts of your experience you want to (and try to) avoid and parts you want to (and try to) cling to. Just watch that too. The only other instruction is to sit as still as you possibly can. If you have an itch, let it itch. If your foot falls asleep, let it tingle. If your back hurts, just let it hurt. We're not trying to comfortable, we're just looking into the full range of our experience for its own sake.