Start the same way you would for any other application:
- Read the Requirements
- Read the Specs
- Think, discuss, think some more
- Write an outline
- Get some feedback
- Write Test Cases
- Send them around for review
- Revise as needed
Check out their white site for white papers, look at slide share for presentations on testing ERP, apply your existing skills & knowledge, look at UPK and the starter kits for detailed test cases, talk to your peers, look at ERP test strategy courses online or in class.
I am currently working in this COE specific to Oracle based apps. I can give you a start here. I was working with applications (Projects) which were developed from scratch so i had requirements/ FD's/ SRS etc which formed as a base to develop test conditions and test cases through which i can perform the RTM or the Tracebility very easily.
When you take Oracle Apps/ SAP or CRM they are considered to be Products not Projects from scratch this differentiates product testers.
There are no requirements as such for these the tester should be a expert in the product to perform testing and to write test cases.
Test Automation tools to be used for Oracle based products OATS - Oracle Application Testing Suite, Open Script, OFT, OLT, PTF, OTM.
I don't build Software but I make them work better, Testing is a Passion