Here a few simple steps to meditate seated on a chair:

-First, sit comfortably on a chair;
-Second, rest your feet flat on the floor and keep legs slightly apart;
-Third, keep your head and your back straight but try to keep the natural curve of your back;
-Finally, your hands can be on your thighs, knees, groin or along your body.

You can try one of these guided meditations.