Like in algebra, depending on how you define the problem, you can get many results that are of little use, no result, the solution you do not like, or finally, sometimes an excellent, easy-to-use result.
So is it about writing down what you have in mind? It is a perfect start because sometimes, what is on one’s mind cannot be put on paper. I will set aside that what you need may be different from what you have in mind.
Your requirements must be read and understood by a number of people who make design choices, and they should prioritize the design choices for the overall project’s success. You must be careful about the exact wording, the logical structure, unnecessary and missing suggestions, and emphasis made between the lines. Requirements should be straightforward to achieve and test in the design while providing success to the project.
Well-written requirements are not a one-go achievement, so without exaggeration, the maturity and success of the product are proportional to the maturity of the requirements defining it. How easy does it sound?