You don't have to think too hard about mindfulness meditation. The good thing about it is that it's easy to do, and anyone can practice it immediately. Basically, all you have to do is to straighten your posture and be conscious of your breathing.

Straighten your posture

You can do it standing up or sitting down. Raise and stabilize your pelvis, straighten the muscles on your back, and gently rock to find your center. Relax your shoulders and face. Lightly close your eyes or keep them half-closed while staring at a point.

Concentrate on your breathing

Breathe in a relaxed and easy manner, and just be conscious of your repeated breathing. The reason for being conscious of your breathing is to distance yourself from worldly thoughts and to connect with physical sensations you're currently feeling. If your focus is drifting to something else, concentrate again of your breathing. Repeat this process several times.
You may find it hard to simply focus on your breathing when you are actually doing it. The trick is to imagine the air your breathing flowing down your throat and passing through your body, and to imagine your lungs contracting and expanding as you breathe.

Time

Generally, mindfulness meditation takes only a few minutes in the beginning. Even five minutes is fine. Once you get used to it, it can be done for longer periods of time, or shorter, at any time and place.

Loose-fitting clothing

Cross your legs and relax your mind. Wearing tight clothing can prevent you from crossing your legs, and the tightness could make it hard for you to concentrate so choose loose-fitting clothing.
Samue, a type of traditional, simple, and functional Japanese clothing allows for easy movement. It is often worn to prevent fatigue.

Don't wear socks  or stockings

In general, mindfulness meditation is conducted barefoot, because socks and stockings can make feet slip, and they can restrict blood circulation.