If you have an error message pop up on your screen, troubleshooting the issue is easier if you have the exact wording of the error. Taking a screen shot of the error will help technical support representatives get to the root of your problem. Screen shots also are useful if you want to keep a picture of a confirmation page when you purchase something online and don't want to print the page. The process for taking a screen shot is the same for nearly all versions of Mac OS on a MacBook.
Have the content that you want as a screen shot centered on your screen. Press the "Command-Shift-“4.” The cursor becomes a crosshair.
Move the cursor to the corner of the screen where you want the picture to start.
Click and drag your mouse to select the content for the screen shot. A selection rectangle appears in the area you define. To adjust this rectangle keep holding down the mouse key as you proceed through the next steps.
Move the selection rectangle as needed by pressing the space bar and moving the cursor. The rectangle follows this movement.
Resize the rectangle around the selection center as needed by pressing the “Option” key and move the cursor. The rectangle resizes.
Constrain the selection rectangle as needed by pressing the “Shift” key while letting go of the other keys. Moving the cursor horizontally constrains the vertical size and vice versa.
Release all the keys, including the mouse. The screenshot will appear on the desktop with the name “Picture” and a number, such as “Picture 1.”
Double-click on the screenshot so you can open and inspect it.
To take a shot of the entire screen, press "Command-Shift-3.”