If I told you that meditating on something was different than thinking about it, you’d probably say that, Duh, you already know that. Yet when people first meditate on the body, that is often what they do: think about it. When you direct your attention towards the physical body, you will feel some body sensation, but you will also experience a lot of mental images of the body. Pictures of the positions of your limbs, mental images of the shape of your torso, and so on. And these mental images are thoughts, not physical sensations.
Meditating on the body means meditating on body sensation, not mental images of the body. So, for example, if you close your eyes and meditate on the bottoms of your feet right now, notice that you can feel the bottoms of your feet, but you may also be experiencing a mental picture of your feet. That imaginal image of your feet is fine and natural, but in meditating on the body, it is important to let go of it. Not push it away, not get angry at it, but simply let it go. Instead, allow your attention to focus on the feeling of the bottoms of your feet. The actual physical sensations. Concentrate on just that sensation, letting go of mental images.
I liken it to a blind man recognizing another person by feeling their face with his hands, his fingertips. Or like feeling a wooden sculpture with your eyes closed. You get into the actual, earthly contact of body sensation; you feel it in the meat, so to speak. Not in your head. In your body. Notice if you can get into the sensation in the bottoms of your feet in that way. There are the pads of the toes, the soles, the arches, and the solidity of the heels. Feel each area of sensation individually. And then feel them all together again. This is actual contact with body sensation. This is meditating on the body. This is where to begin.