To create a custom report you just need to create a data source (sql that will bring the column and information you need in your report) and then create a new report type from where you will call that data source. All this is made in workbench.
After that you can call your report from the front end.
You can open an existing request type (similar to the one you need) and check how is everything called.
I recommend you to follow the steps and examples in the "Reports Guide and Reference" for 9.1 (or the version you are on). I attached the file for 9.1 in my response.
Please check chapter 8 Report Types on page 169.
Hope this helps!
Jonathan (Remember to mark the topic as solved and provide kudos if a post answered your query)