Mistakes to avoid
Functional analysis: expressing needs in terms of functions and methods for finding functions

Don't mix up solutions and functions (waterproofing/protecting against water): always stay at the conceptual level.

Don't forget any functions: all the above methods can be used to list all functions. Use the method that suits you best and check with another method if your list is complete.

Don't over-engineer (too many functions): keep to the user's strict requirements, as each additional function represents an increase in design cost and selling price. Stay at the concept level (no details) in the first instance.