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Use the built-in Snipping Tool application.

1. Click the Start Menu and type "Snipping Tool"
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2. A small window like this will appear
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   1. Once you have this Window open the rest of the screen will have a transparent white mask on the screen. You can go ahead and select(click and drag) what you want to capture.
   2. You can also change the mode do capture a full screen or window snip as well. 
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   3. Don't for get to save your capture. Click on the Save icon (floppy disk icon)
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Mac

1. To capture the full window
   1. Press Command+Shift+3
2. To do a free rectangle capture
   1. Press Command+Shift+4

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1. 2. This changes the cursor to a crosshair, which you can use to select rectangular portions of the screen.
2. To get a transparent window to pop-up
   1. Press Command+Shift+5
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   2. From here you can select to take a screen shot of
      1. Full screen
      2. window
      3. rectangle
      4. record full screen
      5. record window

How to take a screenshot on your Mac (Apple Support).

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