TL;DR work top down System engineer here. After noting any flow down requirements from higher levels (e.g. regulations, safety, security, language, operating

TL;DR work top down System engineer here. After noting any flow down requirements from higher levels (e.g. regulations, safety, security, language, operating system, usability, availability, quality, corporate standards, coding standards, maintainability, etc.), I find it useful to first define the highest level (context) of your system, write out the concept of operation and define the major interfaces and then work down/in from there. A set of diagrams can really help with this process. And a spreadsheet and numbering system helps for traceability. Good luck.