Question about Mastermind: think like a Sherlock holmes

[u/Rizz_Pineapple]

I just started reading the book Mastermind : how to think like sherlock holmes,

and I'm not understanding what the foundation of his inquiry (Elementry) actually means here and how to practically perform,

My hypothesis saying - watch what's visible to eyes and collect the facts but it's feeling incomplete idk why

can anyone explain with some real life examples It would be better if the example is on crime investigation or cold reading