When you click on “Customize” option you will be taken to “Custom Reporting” tab with the current report shown in an editable format. See the two images below, I have marked two sections in the reports that you see and then same sections are marked in the customizable format. When you compare these two views, you will get an idea on how to construct your custom reports.
Custom Reporting View
In the “Custom Reporting” view you can also add remove Metrics and Dimensions, rename the tabs and add filter and save the report as a custom report.