If you just want to do it in MS Word, search for a template online. Lots out there. Other way is to use some kind of tool. Enterprise Architect (sparxsystems.com) that I use caters for use cases. I switched to user stories years ago but it worked in the day. And you can link them to your requirements and create a Requirements Traceability matrix automatically. Lots of other tools will cater for it too I expect

Kimbo