This might not be a useful answer for you, but are you sure any of that is necessary? You can spend months pre-validating an idea and never get anywhere.
Go talk to potential customers now. Find people who are willing to pay for what your are going to build. Once you know they exist, then you can start worrying about market size and the rest. At this point having real life people who want to pay you is infinitely more valuable than any hypothetical data and analysis.
Yes, I totally agree that the best investment of my time would be to talk with customers.
But maybe I am doing the process wrong. I read a lot about how to start by choosing a group and discovering their ideas.
Currently I have some kind of Idea Journal (actually a .md file in a cloud) where I write ideas, needs, problems, possibilities. It might not be the best way to do this.
I was looking for an easy way (from sitting on my desk) to filter them. Initially I get enthusiastic about all of them remembering the context where I wrote them and I think I am a type of person optimistic so I catch myself finding a lot of reasons why most of them are good.
So I read some books about startups, ideas, lean, ... and I got some questions to ask myself about them. This made the filtering process better.
The reason I asked this question was that I wanted to know also more practical examples of what do you - HN - do before taking the first actions (first investments of your time) in the project you want to build.
Go talk to potential customers now. Find people who are willing to pay for what your are going to build. Once you know they exist, then you can start worrying about market size and the rest. At this point having real life people who want to pay you is infinitely more valuable than any hypothetical data and analysis.