How do I break down and plan a personal programming project? [closed]

Question

I've just started a programming job where I'm applying my 'How to code' knowledge to what I'm being taught of 'How to Program' (They are different!). As part of this, I've been taught how to capture requirements from clients before starting a new project. But...

How do I do this for a nebulous personal project?

I say nebulous, as I often find halfway through programming something, I want to expand what my program will do, or alter the result. Eventually, I'm tangled in code and have to restart. This can be frustrating and off-putting. Conversely, when given a fixed task and fixed requirements, it's much easier to dig in and get it done.

At work I might be told "Today/This week you need to add XYZ to program 1" That is easy to do. At home (for fun) I want to make, say, a program that creates arbitrary lists. It's a very generic task. How do I start with that? I don't need it to do anything, but I want it to do something.

So how do I plan a personal programming project?

Related: What to plan before starting development on a project?