What is the correct way to take screenshot for the Mac App Store Submission?
See the docs — in the last row of the table it says:


  One screenshot is required for Mac apps.
  Up to four additional screenshots can be uploaded. Screenshots appear on the store in the order they were uploaded in iTunes Connect.
  Screenshot requirements are:
  
  
  72 dpi, RGB, flattened, no transparency
  High-quality JPEG or PNG image file format in the RGB color space
  16:10 aspect ratio
  One of the following sizes:
  
  1280 x 800 pixels
  1440 x 900 pixels
  2880 x 1800 pixels