There are quite a few getting started guides as well as full guides for the digital mode JS8Call often used for accurate messaging with weak signals over long distances, but I could not find a short concise guide on just the messaging perspective for these purposes.
So I started putting this summary together today and will update and improve it as we use it or other feedback comes in. It can be a useful cheat sheet to print out and have with you for quick reference.
See Use of JS8Call Messages for Exercises or Emergencies | Gadgeteer
Using these messages can be useful during exercises, operations or emergencies to reach others. It is preferable to try to use messaging with acknowledgements for important messages. During exercises, you will probably want to have AUTO+CONF enabled in settings for automatic replies and responses (Settings->General->Networking & Autoreply), and possibly also update your Station Info in settings (Settings->General->Station) to reflect alternative means of contacting your station as this will be broadcast to PSKReporter site as well as to querying stations eg.