In the "Site Admin" menu, select "Custom Reports" under the "Reports" header.
Next to the report you want to run, check the checkbox. Then, at the bottom of the page, click "Run Checked".
When the report has finished running, usually in less than 20 minutes, you will receive an email with a link to directly download the report.
You can also return to the "Custom Reports" page. There, you will see the freshly run report listed and be able to click the "View" button to see it. The Last Run Date will also confirm you are viewing the latest report just run.