Open the Messages app on your iPhone
Find the conversation you want to print
Take screenshots of the messages you want to print
Open the Photos app and select the screenshots
Tap the Share button
Tap Print
Select an AirPrint-enabled printer
Choose the number of copies
Tap Print
Use a Mac or PC if you want to print longer conversations
On a Mac, sync iPhone messages with Messages in iCloud
Open the conversation on the Mac
Choose File
Choose Print
On a PC, export the messages using a third-party tool
Open the exported file
Print from the app or browser used to view it