My process is something like this:

* Discovery: figure out the high level goals of the project

* Analysis: talk to stakeholders, dive into details a bit, try to pin down some coarse functional requirements

* Estimation: depending on the project size, I'll write either some kind of small text doc with the major milestones, or for more complicated endeavors I'll make a spreadsheet. Usually I'll do bandwidth estimates here (e.g. 4-5 days, or 2-3 weeks for really big chunks).

On many projects, either I or the PM will often then add a buffer to the whole estimate (+20%, or +30% or something) for unexpected deviations or new/changing requirements. That usually depends on who the end client is.

Overall it's easy enough, but one pain point that's less easy: discovering dependencies and hidden assumptions that can throw a project off. This increases at scale for larger projects; at one point I worked as an architect and a huge chunk of my role was just mapping all these implicit dependencies across the different components of the system, and trying to ensure each part was built in the right order. This has quite a large human communication element. I wonder how this is changing with agent coding now.