Requirements

Hi all ! I am occupying a product owner position in a medical tech company (first experience).
Shortly our company as one product being a monitoring patch with a web platform for doctors to review patient vitals.

I have been given the opportunity to work on the V2 of the product (especially on the WEB/software side).

While this is exciting I am currently struggling on a specific task.

Since our current requirement/spec system is a mess I would like to start it all over and make things clean.

Initially I thought that things would be a piece of cake. If I have the features and screens in my mind it should not be that hard to convert it into requirements. But turns out it is.

Since I can reshape most of our system I don’t really no which approach is the best for writing requirements and I would like to do things properly to work well alongside the dev team.

Should I write my requirements targeting users ? Should I write requirements towards the system ?
For the same feature being available at two different places should I do two different reqs or one is enough ?
How to segment everything ? Which categories ? Should I divide my system in features or screens ?

If you have any advice, tips or ressources I think that would really help me figure out a good way to do things.

Also we are working on Jira (which I hate but I don’t think I can dodge that unfortunately…)

Thanks !