Access the downloaded file and unzip it using any decompression program.

Go to your dashboard and go to add new plugin

Look at the top and click on upload plugin

Click choose file..find the file and hit open

Hit the button that says install now. The system will upload the file into the plugins directory of your server. Then, it will unpack and install the file into your site. The process is automatic. You just have to wait for your screen to show a confirmation