Then click on the Install App button.
Enter the Apple ID & Password for your App Store account. If you have never downloaded anything from one of Apple's stores (iTunes, App Store, iBooks) then you will be prompted to create an account.
You can launch Battery Health by clicking on the Finder, navigating to your Applications folder, and double clicking the icon for Battery Health.
By default, Batter Health runs in the Menu Bar at the top of the screen. To view the app you must click on the little lightning bolt in the Menu Bar.
Battery Health will show you the current charge level, the health of your battery, estimated time remaining based on current charge for various activities, and details about your battery.