What i'm about to write here will be very difficult to understand. Even i were explaining it to you in person and demonstrating it to you, it is highly likely that you will still not get it. Anyway here goes.
1) most people have a highly simplified view of the human body. A human body is not just one head, two arms and two legs. You really need to look deeper at it.
2) the starting point is standing. Can you stand completely upright or is there a slight lean? When you move, can you maintain the same uprightness or do you start to lean forward?
3) next is sinking directly down, when you sink, do you start to wobble? Can you maintain the same stability when you sink and rise?
4) next is parting legs. When you part the legs, are you able to place 100 pc weight on one leg? When you do this do you wobble? When you sink and rise on one leg do you wobble? When you shift your leg 100 pc to the other leg, do you wobble at any point in the process?
5) next is your upper body, are you relaxed completely? When you hold up your arm does it have any slight tension? When force is applied on you are you able to not resist, and still maintain the touch and not let go? do you have any point in your body where you may unconsciously resist or become tense in certain positions?
6) the above is somewhat simplified but will have to suffice for now. once you have eliminated all the weaknesses in your structure and body as above, the same weaknesses in the opponent's body will become glaring to you.
7) reason is simple, let's imagine you make a lot of effort to stop smoking, and after a few months, you finally succeed. If i now place you in the same room with another smoker, his act of smoking becomes especially noticeable (and probably most annoying) to you.
8) let's imagine now that you are comfortable, relaxed and stable in all of the above movements. This is where the fun starts. When pushing hands, lightly touch the opponent, observe him, ask yourself the following questions, is he standing upright? is he breathing comfortably? Is he balanced and stable? Is there any part that seems or feels hard, tense or immobile?
9) next move your touching arm, observe his movements, does he stick to your arm? how does he react? when he is moving, is he still balanced? does his body move together with the arm? when the body moves is the body still upright?
10) all the above may seem like a lot of work, but once you are used to it it becomes almost instantaneous and second nature. at this juncture simply lead him to make movements which affect his balance or stability. For eg moving foward or backward, rising up or sinking down, turning the waist etc plus any combination of the above. The leading skill does not come naturally and will have to be learnt.
11) a simple example is, if you are erect like this "|" and your opponent is led to lose his stability as well as become like this ">", simply take a step towards him and he will fall.
Bonus edit) most people would wobble when they sink or rise when they have 100 pc weight on one leg. To unbalance him, simply lead him to stand 100 pc on his hind leg (such as executing a roll back), then lead him to rise or sink. At the moment when he moves up or down, apply a light touch at a proper spot. He will lose his balance.