For iOS devices:

iPhone 8 and earlier models, iPad, and iPod touch:
1️⃣ Press the Power and Home buttons at the same time.
2️⃣ Hold them for a couple of seconds and let go.
3️⃣ Open your Camera Roll, which is where screenshots are automatically saved.

iPhone X and later models:
1️⃣ Press and hold the button on the right side of the device.
2️⃣ Then press the Volume Up button on the left side of the device, and then release both buttons.
3️⃣ The screenshot you've taken will appear in the bottom left corner of the screen.

For Android devices:
1️⃣ Press the Power and Volume Down buttons at the same time.
2️⃣ Hold them for a couple seconds and let go.
3️⃣ Open the device folder where screenshots are automatically saved.