Save the installer to your downloads folder and run it.
Go through the installer.
On the page for selecting what to download make sure Unity, Documentation, and Microsoft Visual Studio Tools for Unity are selected. Then select all of the platforms you would like to develop for.
Save the installer in your downloads folder and run it.
Go through the installer and on the page that says Create Desktop Shortcut and Create Associations; under create associations select all 3. Then click Next then Install.