Sometimes a picture of what you see on your computer or mobile screen helps best express what’s going on. Capturing a “screen shot” and attaching it to your support inquiry is really easy and it’s an approach that can put you in the fast lane and get your technical issue quickly addressed.
Mac OS X: Shift+Command+4
Press and hold your Shift key, Command key 
, and number 4 key, and let go. This gives you cross-hairs. Select the part of your screen you’d like to capture.
OS X saves them on the desktop and names them with a time stamp. If you don’t see the screen capture file you made, use your GO menu for desktop.
Apple’s screen capture help page
Windows: Snipping Tool
Press the windows logo 
key on your keyboard. Type “snip” to find and launch the Snipping Tool. Click the down-arrow next to New. Choose Rectangular Snip. Select the area of your screen to capture and then save the file to your desktop.
Microsoft’s Snipping Tool help page
Android OS
Press and hold your power and volume-down buttons at the same time, holding for 1 to 2 seconds. Look to your photo gallery app to locate your screen shot.
A Google Android screenshot help page
Apple iOS
Press and hold your Home button and your Sleep / Wake button and let them go. Your screen shot will be in your Camera Roll.
An Apple iOS screenshot help page
iOS11 Bonus Option
Check out this help page on making a Screen Recording video.