If you want to save a picture of your gameplay or if you want to take a screenshot of a bug that you found, you can take a screenshot by following the steps below:

iOS
1. If your device is an iPhone, press the home button and the on/off button on the right at once.
If your device is an iPhone X, then press the on/off button on the right and the volume up button at once.
2. If you succeed in taking a screenshot, the screen turns white for a moment and the picture is saved in your [Photos] app.

Android
1. Press down the home button together with the on/off button at once. (You may have to press down the 2 buttons for a while)
2. Press down the on/off button together with the volume down button at once. (You may have to press down the 2 buttons for a while)

※Note: Depending on the device that you are using, some Android devices have different ways of taking a screenshot so there are chances that the steps given above may not function correctly. In this case, please check out the User's Guide of your device.