Upload the folder 'wp-popup-scheduler' to the '/wp-content/plugins/' directory. Make sure you upload the whole folder, don't just upload the files. Do not rename the folder or any files.
Activate the plugin through the 'Plugins' menu in WordPress
Visit the 'WP Popup Scheduler' under the 'Options' menu in WordPress
If you are tech-savvy enough, you can edit the CSS file at the wp-popup-scheduler folder to adjust the look and feel. After you have edited the CSS file, place it in your current theme folder. This will prevent any future upgrade from overwriting your popup CSS file.