How do you turn a software idea into a realistic project plan?

I'm learning software development by building real projects. I know the basics of Python, but I haven't built anything substantial yet, I'm not comfortable reading unfamiliar code, and debugging is still difficult.

My biggest problem isn't writing code—it's planning. I have a list of 10–15 web and local app ideas, but when I choose one, I don't know how to move from the idea to a practical implementation plan. For example, I might want to build an app that scans images and extracts information. I understand the desired outcome, but I don't know how to identify requirements, choose a sensible version one, design the basic system, or divide the work into manageable tasks.

AI has made this more noticeable. If I ask an AI assistant what to do next, it may immediately produce an architecture, folder structure, database schema, files, and tests. That can be useful, but it means I'm following a plan instead of learning how to create one. I'd prefer to handle the planning myself and use AI mainly to help implement planned tasks, explain unfamiliar code, identify mistakes, and write or improve tests.

The workflow I'm trying to learn is something like: idea → requirements → scope → design → tasks → implementation → testing → review.

For experienced or self-taught developers:

- How do you turn an idea into a plan for a small application?
- What do you write down before coding, and how do you know when the plan is detailed enough?
- How do you break features into tasks that are small enough to implement?
- How do you decide what to leave out of the first version?
- Which parts of development should I deliberately practice without AI?

I'm looking for concrete workflows and examples rather than prompt-writing or tool recommendations.