At first you can close all your opened projects. It is not necessary but it is a good practice before messing with plugins.
Next from main screen go to plugins configuration page (main screen of IntelliJ -> Configure -> Plugins).
Then in the bottom of the window click button Install plugin from disk....
Now select downloaded zip file with plugin and confirm your choice.
Restart IntelliJ IDE to make changes applied. After that your editor with plugin will be ready to use.