A Summary of the Srimad Bhagavatham : Ch-5. Part-11.
5: Narada Instructs Yudhisthira on Ashrama Dharma :
Here, in these Aranyakas, the various upasanas are prescribed: how the cosmic prana can be meditated upon, how the cosmic mind can be conceived, how Brahma—the Mahat, or the cosmic intellect—can be brought into the focus of our attention, how we can intensely feel the unity of the parts of our physical body with the parts of the physical universe. This is the highest form of upasana that we can think of.
There are also various other ways. This is a transcendental technique of the Aranyaka portion of the Vedas, but we have other devotional paths which can also be called upasana—such as contemplation/meditation on a form of God, or an ishta devata, as it is called, that we think is suitable for us. The ishta devata is a chosen deity. It may be the name that we give to our concept of God as a person pervading the whole world, or as a person seated near us as an image on our altar or a murti in a temple, as the case may be. In the earlier stages, we may require a physical form of the object of our meditation, and that could be a yantra, mantra, murti, image, idol, saligrama, painted picture or whatever it is, for the purpose of concentration.