In the accessibility settings, scroll down to the bottom of the list.
Tap on „Guided Access“.
Tap on the switch to activate it.
Guided Access is now avtive, and you can now change some of the settings.
Once you are done, tap on the Home-Button to get to the Home Screen.
Now open an app.
Now, quickly tap the Home-Button three times.
Now a „Guided Access“ Overlay appears over the app.
To activate Guided Access, tap on „Start“.
You now have to input a passcode.
Now, you can‘t close the app.
To quit Guided Access, press the Home-Button three times again.