I'm a professional software product manager and have defined, built, and deployed dozens of software products over the years. I've set up and managed agile development teams using both onshore and offshore developers using SCRUM and have participated as product owner and scrum master.
The work you are asking for is not something that can be done in a single day. Creating a product specification and development plan takes a while and the rule of thumb is "The earlier in the development process defects are found, the cheaper they are to fix." Meaning, find and fix problems during requirements definition.
I'd be glad to help you but it needs to be understood that it is a non-trivial task and must be done right if you want a successful product,