How do I select the report to use as a basis for a new custom report?

I want to create a custom report. How do I select the existing report I wish to copy for using as a basis for the new custom report?

It is very important to identify an existing report which can serve as the basis for the customized report you wish to create. You must take into consideration when choosing the report both its layout and its data structure. Every report has predefined data source relations and fields that can be selected.