@Val ******
very easy to do at a local immigration office. You need a passport photo, a copy of your passport data page and a copy of your entry stamp. You fill out a form called a TM7 and pay 1,900 baht. You’ll get a stamp for another 30 days. Depending on the office you use it might take somewhere between 10-40 minutes.
If you mention what office you’d use then we may be able to advise whether you’ll need to have a TM30 filed. Most offices don’t require this for an exempt extension but some are quirky.