Ask HN: Planning a software project

What would be an online source for learning the basics of software project management?

My usual projects only involve myself, so I haven't had the following issue so much.

At work, I'm responsible for a software project.

I am the only programmer (although I'm more a researcher in this role). I have to manage expectations with my boss.

There's a time limit to the project.

There's a vision for how this 'big' system would work, but a small prototype I created already exists.

It's difficult to find a way to break down the features being suggested, with the amount of time they would take. Writing little descriptions in a spreadsheet row doesn't seem to be descriptive enough.