Mindfulness of body is not just in reference to the outer form, but to energetic sensitivity. Data from the body is direct, not filtered through the mind. If we track it and tune into it, it can release blocked areas, clear psychological effects and bring clarity. Approach with goodwill and softness of attitude.
Developing true well-being begins with the heartfelt intention and commitment to do so. Mindfulness is the basic tool that will help manifest that intention by weakening unwholesome states, cultivating wholesome ones and amplifying the latter when they arise.
Sanna (pere arise together with feeling (vedana) and is a "marker" of sign and a "recognition" of the mark (sign). It can be used to protect/develop and understand mind's dynamic. What is needed for a beginner = mindfulness of the body.