In the settings app, go to “Security”.
Now scroll to the bottom.
Then tap on „Screen pinning“.
Now, you can activate screen pinning by tapping on the switch.
Screen pinning is now active.
If you don‘t want people to exit a pinned app without the unlock pattern, also activate that option.
Then, press the recent-apps button, to see all currently open apps.
You will now see a pin symbol in the bottom right corner of each open app.
If you tap on the symbol, this app will be pinned to the screen.
Tap on „GOT IT“.
Your app is now pinned, and you can‘t close it with the Home-Button.
To close the app, press and hold the Back-Button and the Recent-Apps-Button simultaneously for a second.
You are now on the lockscreen, and can unlock your tablet with your unlock pattern as usual.
Once you have unlocked the tablet, the app is no longer pinned.