1: You follow a link on your phone that opens the app, you're prompted with the transaction sum and then sign with a code on the phone and done.
2: You scan a QR code (from sign or screen), opens a prompt in app for the sum, sign and done
3: Person-to-person, you enter their phone number(often from the phone book), amount and message, sign and done.
The signing is done via the standard app that's also used to login to the tax-office,etc so the entire population has it.