How do I report 90 days in Thailand?

If it is your first time filing a report for 90-days you need to go to the Immigration but if its your second time filing a report you can easily do it via the Internet by yourself as follows: Enter the website of the Immigration Office at URL : https://www.immigration.go.th/ and select the Online Service (Over 90 day).

Note : The online application must be submitted within 15 days of the due date. For example, if the due date is 27 September 2020, you can submit the application from 13 September 2020