On your created custom report's Editor. You will see multiple tabs [Editor, Audience, Schedules, Access]. To schedule automatic generation (sending or receiving) of reports go to:
Step 1 : Go to your Custom Report > Edit
Step 2 : Click Schedules > New schedule
Step 3 : Give it a name and choose its format ( .csv by default);
Step 4 : Decide when the report will first be sent out and how often to send it.
Step 5 : The audience will already be selected if you have defined your audience.
Step 6 : If desired, you can add a message to the recipients.
Step 7 : In Advanced, decide what to do if there is nothing to report.