Open the App Store on your iPhone or iPad
Search for iMessage games or game apps with iMessage support
Download and install the game app
Open Messages
Open a conversation
Tap the App Store icon next to the text field
Tap the app drawer icon if needed
Select the game app from the list
Follow the on-screen prompts to start a game
If the game does not appear, check that iMessage apps are enabled in Messages settings
Update iOS if the game still does not show up