1. Install the app through the Weebly app market:
https://www.weebly.com/app-center/comments-plus
2. You will be asked to authorize the app. After authorizing the app, the Weebly editor will open for you.
3. In the Weebly editor, open the page where you want the Comment box to be installed
4. Drag the Comment app icon from the side bar onto the page:
5. The App's settings window ("Comments Editor") will open and let you customize your comments box. Don't forget to Publish your site when you finish.