Keep clicking past the license agreement, etc., until you get to the screen that asks you which disk to install OS X on. Once it has been downloaded, just double-click the installer, which will be located in the Applications folder. Note that if you already have that version of OS X installed, you’ll see a popup message appear asking if you still want to continue or not.
Go ahead and click the Download button to start downloading the installer. Once you open the App Store, you’ll see a link on the right for the latest version of OS X (El Capitan as of this writing).
The easiest way is to download OS X from the App Store. I’ll show you both methods in case one isn’t working for you. There are two ways you can install OS X on to your external hard disk: by reinstalling OS X from the OS X Utilities repair screen or by downloading OS X from the App Store and running the installer. It should only take a minute or two for the drive to be erased and reformatted. Give your drive a name, choose OS X Extended (Journaled) for Format and GUID Partition Map for Scheme.
