After the zip file has downloaded:
In the IntelliJ IDEA Settings/Preferences dialog, select Plugins.
On the Plugins page, click and then click Install Plugin from Disk.
Select the plugin zip file and click OK.
Click OK to apply the changes and restart the IDE if prompted.
You can also drag the downloaded plugin zip file and drop it into the IntelliJ IDEA Welcome screen. (The Plugins page is also accessible from the Configure menu on this screen.)