Depending on the device you use, there's different ways of doing it:
For Android devices:
- Press the On/Off + Volume down buttons for a few seconds.
- For some devices, mainly for those devices with older Android or Samsung versions, those buttons may be the On/Block + Home buttons.
You can check if the screenshot was taken by going to the gallery, more specifically to the "Screenshots" section. Also bear in mind that, when a screenshot is taken, the camera makes a noise so, if you heard it, that means that the screenshot was taken successfully.
For iOS devices:
- Tap on the Block (by the side of the device) and Home (at the center of the device) buttons at the same time. If you're not able to take the screenshot, keep trying.
To find the screenshot, go to the device's Gallery, then to Albums and go to the Screenshots folder.