To state, custom reports are not supported.
With that said, to create a report from blank template, you will need to know the sql query to obtain the data, and once that is accomplished and viosually prepared, CCA will not be able to execute it, it will need to be run from Jasper itself (Jasper Studio more times than not, unless they ahve Jasper Server in their environment where they can put the reports). CCA uses a Jasper library file, not Jasper Server
Now, if they want to tweak \ modify and existing report, I created a tech tip on how to do it: Tech Tip: How to modify existing CCA Reports with Jaspersoft