Define your newsletter premise in one sentence
Before writing anything, answer three questions: who is this for, what do they get, and how often. A strong premise sounds like "a weekly newsletter for freelance designers sharing client management tips and industry observations." A weak premise sounds like "a newsletter about design." The difference is specificity.
Your premise determines everything: what you write, who you invite to subscribe, and what your signup page says. A vague premise produces vague content and attracts an unfocused audience that eventually stops opening your emails.
Write your premise sentence before touching any writing tool or email platform. If you cannot write it in one clear sentence, the newsletter is not ready to exist yet.