The Mahāsatipaṭṭhāna Sutta — the Longer Discourse on Mindfulness Meditation — is the Buddha's most detailed instruction on what to attend to and how. It is the twenty-second discourse of the Dīgha Nikāya, and it survives in a near-identical shorter form as the tenth discourse of the Majjhima Nikāya.
It sets out four things to observe: the body, feelings, the mind, and principles. The instruction repeats the same shape for each, which is what makes the discourse read as a method rather than a description.
This site publishes the discourse in full, in Bhikkhu Sujato's translation, with the Pali alongside. A second section covers the Buddha's life and the basic terms of the teaching. See About this site for where every text comes from.