Sit in a chair with a straight back, sit on the floor or a comfortable pillow, or you can lay down on a comfortable bed or couch. Set a timer for however long you'd like to practice for. Ten to fifteen minutes could be a good time to start with. Alternatively, you can try using a guided meditation for your practices (see Additional Resources for options).
Consider the movements of your body as you breathe. Feel the sensation of your breath enter your nose and mouth, and exit as you exhale. Consider the movement of your chest as you inhale and exhale.
Notice the sounds you hear around you. Notice your thoughts as they pass through your mind. Feel the sensations of the wind across your skin, or the weight of your body on the surface you sit upon.
As thoughts pass through your mind, notice them. Acknowledge and accept these thoughts, without judgement. Then let your thoughts pass by. If this becomes difficult, focus back to your breathing, and then widen the focus again to include your thoughts and environment, as soon as you're ready. If you've set a timer, when it goes off, your practice is done, unless you decide you'd like to continue.
Consider adding this or similar mindfulness practices to your daily routine. You could practice mindfulness when you first wake up in the morning, or before you go to bed at night. You could take ten minutes when you get home from work, or practice mindfulness as you eat your lunch. Or, you could follow the guidelines of many of the books published by experts on mindfulness, such as the various eight week programs.