The words Kayanupassana, Vedananupassana, Cittanupassana and Dhammanupassana mean being 'closely observant' of the body/breathing, feelings, mental states and realities.
These four objects of close observation occurringoccur naturally & automatically when the mind has been sufficiently developed in quietude, purity and freedom from craving.
Therefore, how to start and how to do it is simply by stopping craving & making the mind quiet.