Write Some Use Case Descriptions
In step three, you may have generated ten to fifty use case names on your first pass. That number will grow as you continue to formalize the software requirements specification. That level completeness of the specification is very desirable because it gives more guidance in design and implementation planning, it can lead to more realistic schedules and release scoping decisions, and it can reduce requirements changes later.
The downside to mapping out the big picture is simply that it is too big. It could take a long time to fully specify every use case that you have mapped out. And, the resulting document could become too large, making it harder to validate and maintain.
The solution is to be selective before moving to the next level of detail. For example, if there are clearly too many use cases for one release, reschedule some of them for later releases. Also, it's a good idea to just write descriptions rather than get into detailed steps for each use case. Going deep into the details of just a few use cases is enough to shake out uncertainties and identify areas for improvement. The bulk of the use cases can be done later, it needed at all.
Write one to three sentence descriptions of each use case that you plan to implement in this release. The description should provide a little more information on the user's goal and briefly outline the strategy that the user will follow. Sometimes there will be two or more ways to accomplish the same goal. If there are significant differences in strategy, it is a good idea to split the use case into two distinct use cases.
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