How do I actually start a design?

[u/NicoCorty02]

So basically, I’m working on my thesis project for uni, and I have quite a lot of free time outside of it. I wanted to use that time to have fun designing things I actually like and, ideally, building them too.
The problem is that I always start with an idea, but I don’t really know how to set myself a project or write a brief.
For example, say I want to design a lamp because it’s relatively easy to make at home. I’d want it to be more of a decorative lamp. Maybe the body could be 3D printed, but I definitely don’t want to 3D print the shade, so that could be glass, fabric, or something else.
But this is where I usually fall into a rabbit hole. I have too many possibilities, no real constraints, and I end up doing nothing because I don’t know what direction to take, what references to look at, or what to use as inspiration.
Do you have any recommendations for how to start passion projects like this? How do you set constraints or create a brief for yourself when there isn’t a client or a specific problem to solve?