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 well of this I'm taught how to capture requirements from clients, so as to start a new project.

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 programme from a -> b.

So how do I plan a personal programming project?