If you're not already familiar with SQL, get to know the basics. Custom "reports" are just SQL views: queries, pulling data from the multiple tables that make up the LANSweeper database.
Khan AcademyW3 SchoolsMS TechnetTake a look at reports that came with LANSweeper to help get an idea how the tables relate to one another.
Use the "View Database Documentation" link at the top of the report editor to get to know what data elements are stored in which table.