In a nutshell: Write the instruction half of a prompt so a model follows it
Use it when: You are writing any prompt and want the rules about instructing a model in one place, rather than restated in every one.
Put what the model is to do at the beginning, before the material it applies to. The guide records this as a recommendation rather than a law: "Others recommend that you place instructions at the beginning of the prompt."
Put a clear separator between the instruction and the material it acts on, so neither is read as part of the other. The guide's own example separator is ###, used "to separate the instruction and context."
Write the behaviour you want rather than a list of prohibitions: "avoid saying what not to do but say what to do instead". The guide's own example is a recommendation agent that asks for the customer's interests immediately after being told not to.
Prefer a concrete instruction to an elaborate one — "It's often better to be specific and direct." The guide contrasts asking for an explanation that is short and not "too descriptive" with asking for two to three sentences pitched at a high school student, and prefers the second, because the first says neither how long nor in what style.