Step 1:
At the top of the screenshot editor window, click the Arrows button.
Step 2:
On the right, choose the style of the arrow:
-
Choose an arrow shape. The Basic group contains straight arrows, and the Tapered group contains arrows that are wider towards the head.
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Under the shape styles, set the color, width, and outline for the arrow. You can also change these after you've added the arrow.
Step 3:
Click Add Arrow to place the arrow onto the image.
Step 4:
Move and resize the arrow on the image:
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Drag by the green arrow circle 
to resize and rotate the arrow.
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Drag by the arrow shape to move it.
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To delete the arrow, click the green circle with the cross 
.
