Assume a lotus position posture for this meditation.
If you're not able to do a lotus position, sitting in a straight-back chair with your legs in front of you, feet on the floor will also be fine.
Keep your eyes open for the meditation - stare at a blank wall or door. (Note: it may be easier when first learning zazen to have your eyes closed - it's ok.)
Inhale slowly through your nose. Focus all your attention on the sensation of inhaling. Feel the coolness of the air as it enters you and fills your lungs. Hold that breath for a second or two.
Exhale slowly through your mouth. Focus all your attention on the sensation of exhaling. Feel the warmness of the air as it exits. Hold that lack of breath for a second or two.
Now, inevitably, thoughts will enter your mind, though you're only supposed to be focusing on your breathing. That is to be expected, so don't worry! When you find yourself thinking, note the thought, and then release it. Every time a random thought enters your mind, note it, and release.