Welcome to the WGT, glnowman!
To Swing: Left click the swing meter at the bottom and hold the mouse while you drag it forwards. You'll see a yellow power bar increase to the left. Continue to push the mouse forward (still holding the left click) until you get to your desired power, then let go of the button. You'll see the yellow bar will slide to the right. Try your best to stop it on the fat white line by left clicking again.
For a more in-depth video tutorial: Click Here