Requirements: jailbroken iPod Touch, WiFi connection.
If you already have MakeItMine, you can skip to step 4.
1. Connect to a WiFi hotspot, then open Cydia. Let it update the sources and load the homepage.
2. Tap the Search button, then search for MakeItMine.
3. Tap the Install button, then the Confirm one. Then wait until Cydia downloads and installs MiM. When finished, respring if needed, if not close Cydia.
4. Now open MakeItMine (MIM on the Springboard). Type your message in the text box, and press either the Carrier button to change the iPod text, or Banner to change the statusbar clock. You might need to respring after changing the banner.
5. All done!