1. Download the theme and extract it in your computer, then navigate to x/framework/plugins
2. Extract cornerstone.zip in your computer
3. Login to ftp using filezilla – https://filezilla-project.org/download.php?type=client
4. Navigate to your extracted cornerstone file in local site and navigate to wp-content/themes/x/framework/plugins in remote site
5. Upload the unzip cornerstone folder to wp-content/plugins in your remote site
To install plugins manually guide, please refer to the video: