Gil Fronsdal instructs the alert posture before starting the guided body scan meditation. Taking long slow in-breaths connect with the body and relaxing.
He starts with instructions to a proper seating posture. Starting with deep breaths, being aware of the body as you inhale and then letting go and relaxing upon exhaling.
Feel your body, become familiar with the body as if it's from the inside. How does the body feel itself?
See if you can notice how the rhythm of breathing, sensations of breathing, impacts the body.
The process of mindfulness is not about judging what you're aware of, but by being familiar with your experience, with your breathing.
Then familiarizing yourself about your body, without judgement but just loving recognition of how your body is. Tired, relaxed, tightness. Being mindful of how things are is central to the practice. Part of being mindful is how we're aware. To hold every experience in our hearts.