This is how I work on my projects as an indie dev. When I start working on something significant (a new feature, for instance), I'll create a markdown file that has a summary of what I'm trying to achieve and then a TODOs section which turns into this massive outline of all the tasks that I'll need to do to complete the work.
At first, the outline just has a few tasks that are fairly high-level, but as I dive into each one, I add more nested sub-tasks. The nesting keeps going until I end up with sort of leaf nodes that can be done without depending on other tasks. This gives me a nice visual of how complex some tasks are versus others.
I generally prototype and design the implementation at the same time, and having this outline gives me a place to "dump" tasks and other work I'll need to do later, and you do often encounter more work than you expect, so an outline makes it easier to find a good "parent" for the task. Having a big outline also lets me jump around from high-level design to low-level implementation easily as well, which you need if you're prototyping and trying to find the right shape for your solution.
It's great as a motivator too since I can see when I complete something big when I check off a parent task that has a lot of nested children.
I find a simple text file outline like this is so much more convenient than say an app or a web UI since I can just jump around the file and cut and paste outlined sections and indent or un-indent them to re-parent them. (Having to use something like JIRA to do this would be way too slow, especially when you're in a flow state.)