Sitting meditation is the heart of Zen practice. When we sit in meditation we try to keep the spine straight, our eyes open and our hands in our lap forming the maha mudra (Great Hand Position). We also sit completely still trying to refrain from fidgeting, scratching, and switching positions. At the beginning this may seem difficult but with practice it is possible to control this body. When we can control this body it is easier to control our minds.
Even though sitting still with good posture is important, the true point of meditation is to pay attention to our mind. True sitting means to cut off all thinking and return to this moment. For this purpose, there are many techniques such as mantras, counting breaths, or keeping a question. The important part is not what technique you use, but to always come back to it if your mind wanders away. This coming back is true meditation. This is possible to do even after the meditation session is over. When you can keep this mind in your day to day life, then any activity becomes Zen practice. When driving, only drive! This is driving Zen. When eating, only eat! This is eating Zen.
For a more in-depth explanation of the mechanics of sitting click here.