Once logged into your Geotab database, click Administration > Reports... > Report Views > (select the report you're interested in modifying by clicking on Preview) > (click on the excel icon to download report) > (open excel file downloaded from the previous step) > (add custom columns).
* Keep in mind that you will need to work with the data that is available to you within the downloaded report.
** For advanced custom reporting you will need to merge two or more reports together to increase available data.
After the custom report has been created, to upload it back into your Geotab database, you can drag and drop the file from the downloaded location directly into your database by performing the following: Administration > Reports... > Report views > Add Excel File > (drag and drop file into the labelled area).
* Keep in mind when uploading a custom report that the report cannot contain more than 1000 rows, or else it will give an error.