How to use existing database for custom reports:

When you customize an existing report, or create a new one, you will use monitoring data that is stored in the database you built during the setup, and that is fed regularly by the data of all the servers and websites that you monitor.

You may see 2 types of data that you can use : - tables, which are "pure" data as they arrive from the servers you monitor
- stored procedures, which are an aggregate of many tables, which have been made for the need of a report or a dashboard

You may use one or the other, according to your needs. If you are used to SQL Server Management Studio, you can even create your own stored procedures, and use them on your customized reports.