Here's how you can create a follow-up to a proposal:
1. Go to the proposals tab in the top header.
2. Click on the double arrow icon below the proposal you want to follow-up on.
3. Schedule a follow-up with any notes.
On the day it is scheduled, you will be emailed a reminder and it will pop-up on the home screen of the web app as shown below.
To clear it out, simply click "(Dismiss)".