Check to see if the people you are working with have texting functionality. Most phones do. Many other devices do, as well. Not everyone likes texting, but be prepared to show the basics for those who want to know how to text. Many businesses, including doctor's offices and pharmacists, now offer text reminders.
They should know how to input text from the keyboard on their smartphone or tablet screen. They'll have to understand that to get to different cases, numbers, and some symbols, they need to change back and forth between cases. You can also show them optional characters they can use, like emojis.
Typing with your thumbs on a small screen can be difficult. Some people may benefit from using a stylus designed for phones or tablets. Some computers contain apps that connect to the texting app through a person's phone. That way people can text using a full-size computer keyboard and monitor. Some people may appreciate doing that. Remind them that texts are usually short, however. Longer messages should probably be sent by email or by calling.
You may want to show people how to enter text with their voices for ease of use. They'll have to understand that it's not always 100% accurate, but voice typing can be a fast and effective way to compose a message. Let them know they can use their keyboard to go back in and edit messages before they're sent. Show them shortcuts, like pressing on the message window on an iOS device to simulate a cursor. Android phones use arrow keys.
A skill many people will find helpful is saving pictures from a text to their Photo gallery. You can teach them to do the opposite, too. Show them how to send a picture in a text, either from their photo gallery or attaching it through the texting keyboard.
You may want to show people how to block an unwanted sender in text messages as well, just like with calls. Spam sent in a text is becoming much more common.