Mistakes to avoid
Functional analysis: classify and characterize functions - draw up functional specifications

Give yourself too many constraints: in this operation, the user's needs are translated into figures, measurements and tests; be realistic and translate only what the user wants.

Think that the user can't measure anything: on the contrary, the user has a very good perception of what he wants and can therefore verify what you may be claiming in your sales pitch. Never display arguments that you are not sure will hold up over the product's lifetime.