One sentence that says what the app does for whom.
A second sentence that names the problem it removes, in the words the user would use. No feature list yet.
Feature one, as a benefit, in five words.
Two or three sentences on what the user gets. Say the outcome before the mechanism, and say the mechanism only if it makes the outcome believable.
Feature two, as a benefit, in five words.
Two or three sentences. If this feature only matters to some users, say which ones — a narrower promise is more believable than a wider one.
Feature three, as a benefit, in five words.
Two or three sentences. The third feature is usually the one that answers the objection the first two raised.
How it works
Step one, a verb.
What the user does first, in one sentence.
Step two, a verb.
What happens next, in one sentence.
Step three, the result.
What they have at the end, in one sentence.
Be first to know when it ships.
Restate the promise from the top in different words.