IOS:
  1. Press and hold the Sleep/Wake button on the top or side of your device and the Home button at the same time.
  2. To find your screenshot, go to the Photos app > Albums and tap Camera Roll.
  3. Go to the open in-game conversation, tap on the attachment icon next to send button to add the attachment
ANDROID:

For the vast majority of Android phones and tablets, to take a screenshot, you simply press and hold the Power and Volume-down buttons together.

However, you can also check out this link to find more device-specific instructions: http://www.androidcentral.com/android-screenshots