Here are some key guidelines to writing a software requirements specification (SRS)"
1. Requirement Modeling asks, "What", not "How". What user interactions are involed, what objects are manipulated, what functions must be performed, what behaviors does system have, what interfaces, and what constraints apply?
2. Keep in mind the 3 goals of the SRS:
a. Describe 100%
b. Create enough information for the next step: software design. This information may include sample user documents currently used such as excel sheets, printed reports, etc. shown with the requirements description or placed in appendix section.
c. Define requirements that can be validated as "done" once design is completed and again when software is built.