As with most things APRS, there are multiple ways to accomplish a task. In this case, there are many ways to send messages through APRS protocols. Everything from smartphone apps to DTMF capable radios. All of those various modes are detailed here. Because this is an introductory guide, we are going to concentrate on the straightforward radio-to-radio abilities.
First, refer to your radio’s manual on how to enter the messages. The feature is usually quite straightforward. If your manual isn’t that straightforward, a quick YouTube search for your radio model number and the words APRS Messaging will likely yield a tutorial.
Before you try to message someone via APRS, it’s useful to make sure their radio is online first. If you are receiving their signal via RF, then you know they are within simplex range. If they are not in simplex range, you can still message them! It will either be digipeated, passed through in iGate, or both. Just check one of the APRS Global Internet Systems. Personally I use http://aprs.fi/. Just enter their callsign and Secondary Station Identifier (SSID). If you see a recent packet from them, then there is a good chance your message will reach them. I encourage you to check for me! If you see me beaconing my packets from Thailand, go ahead and try to send me a Hello, Aloha, or Swatdii khrap!