My recent workflow has been this:

1. Put the project in a private GitHub repo. Paying for it adds incentive to work on it.

2. Break up the work into small tasks and add them as issues in the GitHub project. Aggressively separate them into milestones to stay focused (Milestone 1 == MVP, less important stuff in Milestone 2, etc).

The reason I think this works is that it is easy to get overwhelmed by all the work on a side project, and you have limited time to work on it. Now when you have time, you grab a ticket, complete it. Each ticket should be doable in a few hours or the scope of your ticket is too broad.