These three forces are what we seek to unify when we do all of our training. It is easy to put our mind in connection with the human force, the mix of yin and yang energies, since it is the closest to us and our physical being.
Once you have a grasp of the human force (energetic system), expand it upward and downward into the heavens and the earth. Try to reinforce the connection between each of the three forces with every breath.
The heaven (yang) and earth (yin) forces take practice to connect to, and even more practice to stay in touch with. I suggest practicing on connecting to one at a time during your daily practice and meditations. Over time, you will notice the different sensations that are associated with the different forces, and how your own energy combines them all.