Have you zeroed in on an ERP solution? Documentation will be provided by the vendor implementing the system. Have you done an analysis of your requirements for the HR system? The first step is to make sure what you want and what is provided by the system. The gap analysis between these is important. Then, how will the vendor customize the solution for you? Make sure you have a target date in sight.
Based on your discussions, the vendor will provide an SRS document specifying what the system will do and what it will not do. After this has been agreed upon and the vendor develops the system, then you can think of implementing. You must see a demo of the system based on the SRS to make sure you are getting what you want.
Regards, Ronald