If you haven't applied for a tourism extension on your current visa-exempt stamp, you can apply for a 30-day tourism extension for 1900 baht fee ONCE.
You are going to go to IT Square Laksi Plaza temporary office, just next to Lak Si station of SRT Dark Red Line.
You will go here.
****************************
You can book online here if you want. They accept walk-ins.
*****************************************
Attached is the general information for the tourism extension.