Doubt with Kreports Module (SuiteCRM Version 7.4.3 Sugar Version 6.5.20 (Build 1001))

Good afternoon,
i am doing a internship in a company named G9Telecom localized in Portugal. The main goal is to create KPI´s that monitor the comercial activity, and i am using the Kreports v3.0 module to do that. However taking into account that i would like to do some calculations using the “CustomFunctions”; thus i would like to know if is possible to give me some tutorial or guide where i can learn how to use these functions.