The mini has 4 buttons; Up, Down, Ok, and Back (plus SOS and power, which aren't relevant for this part). From the main menu, go down to Messages. Hit Ok. Select/start a convo, using Up/Down/Ok. Use Up, Down, OK to select letters, once you get started you can hold Ok down for a moment to use the predictive text. With about 5 minutes of practice, the mini is easy to navigate and sent basic messages from. I would not want to send more than a handful of messages per day this way if they were long, but short messages are easy enough. Being that phones break/die/etc, you should take a few minutes and learn your SOS device a little better, trying to figure it out in a stressful situation is a recipe for disaster.
When you're not actively using your inReach, I highly recommend locking the screen (hit the power button, then Lock Screen). This will save battery life and will let you return to whatever screen you were using last.
