[blcArmadillo]

I know you said you're not asking for a specific tool recommendation but it sounds like you want something like Trello (trello.com). Basically you first need to come up with the individual tasks that are needed to complete the project. At first you'll likely miss things but as you're developing you can add new tasks. For each of these tasks you'll create a card. Then on the Trello board you'll have vertical columns called lists. A simple set of lists would be:

  - Backlog: Tasks that haven't been worked on
  - In progress: Tasks you're working on
  - Complete: Tasks that are finished

You can always add more lists if your workflow requires it (e.g. adding a "In testing" list). All your tasks/cards will start in the "Backlog" list. It's also a good idea to roughly order them in priority. You can then pick the highest priority task and move it into the "In progress" list. Once you've completed the task you can move the card to the "Complete" list. Then grab the next highest priority task from the "Backlog" and repeat the process over again.

When you come back from not working on the project for a while all you have to do is look at what task(s) you have in the "In progress" list. If that information isn't enough to figure out where you left off, the task is likely to broad and should be broken down into more detailed tasks. After doing this a few times you'll get more used to creating tasks at the appropriate level of detail--it's just something you have to learn from doing.