Adaym,
I think you need more practice and experience than formal schooling.
Internal audit reports should be relatively brief. The auditees should not be learning much from the report, they should already know how things went and what was found. No surprises in the audit report!
My summary reports include participants, strengths, OFI's, observations, and major and minor nonconformities.
The latter 2 items are what are most often poorly written, IMO. I list the requirement, the finding, and the objective evidence supporting the finding. Use the golden rule -- make your audit reports look like what you would like to see given to you.
You could write up a hypothetical report and submit it here for critique.