Download the plugin from the source (zip file). Go to WordPress admin area and visit Plugins Add New page.

Click on the Upload Plugin button on top of the page. This will reveal the plugin upload form. Click on the Choose File button and select the plugin zip file. After you have selected the file, you need to click on the Install Now button.

WordPress will install it for you and you will see a success message Once installed, click on the Activate Plugin button.