If you entered on the visa-exempt scheme for sure, you are going to go to IT Square Laksi Plaza temporary office, just next to Lak Si station on SRT Dark Red Line.
You will go here.
****************************
And you can book online.
********************************************
*****
Attached is general information.
You don't have to have a flight booking because Bangkok Immigration just doesn't ask for it for a 30 days tourism extension, not because of your entering by land border.