In order to install your downloaded WordPress plugin, login to your WordPress Dashboard and go to Appearance → Plugins. Click on ‘Add New’ at the top of the plugins screen.
You are able to search for WordPress.org hosted plugins now. However, you want to upload a downloaded plugin, therefore click ‘Upload Plugin’ at the top of the screen.
Then select your downloaded plugin package (.zip Archive file) and click on ‘Install Now’. WordPress will then automatically upload the zipped plugin file, extract it and install the plugin.