If you are relatively new to sitting meditation practice, it may help to consider the following: Create a comfortable, private space to sit. It could be a corner of your bedroom, a walk-in closet or wherever you can have privacy. After even only a few days, the consistency of sitting in the same place will lead to a greater ease in letting go of other concerns and settling into 'just sitting.' The same thing goes with time: it's best, if possible, to sit at the same time(s) each day.
Sit so that your knees are lower than the top of your pelvis (the iliac crest). If you are sitting on a cushion, sit toward the front of it so that your pelvis tilts slightly so that your lumbar spine maintains its natural curve. If you are sitting on the floor, and your knees do not come to the ground, bring the ground up to your knees by supporting your outer thighs with rolled blankets or yoga blocks. You want to feel completely supported. As Patanjali describes asana, your posture should be stable and easeful.
Let your jaw be relaxed; perhaps your lips are even slightly parted. Eyes can be closed or partially open, but in either event, have your eyes rolled downward, as this helps stimulate the parasympathetic nervous system responsible for the relaxation response.
Rest your hands on your thighs, palms up or down, whichever you prefer OR use a traditional meditation mudra.
Decide before you sit how long you plan to sit and then sit there that long! Don't cut it short 'because it's not working' or because you're bored or think of something better to do! If at the end of the time, you'd like to sit longer, that's fine.
Sitting quietly, feel what is sitting. Feel the body 'in the body.'
Observe the ever-changing sensations and see if you can stay at the level of sensation and not rush to judge or mentally describe them.
Go into the sensation and see if you can realize the subtle presence of that which senses. Feel the sensation within the sensation.
Rest in just sitting.
Can you locate that which senses? Is there a defined boundary of that which is aware?
Let this be simple. Don't try to be 'spiritual.' Just sit free of any inner seeking for attainment. Let go of strategizing and simply be present to your life breath by breath. Practice radical acceptance.
Remember, there's room for all of it on your cushion (or chair).