How can I extend my stay in Thailand after my visa on arrival?

Better go to immigration office as soon as possible, There is no downside to apply early your 30days extension is added to your current stamp. You wouldn't lose any day. Just make sure you get 30days when you get your passport back.

I'm currently at immigration office in Bangkok now. Super easy to do. Fill out a form, make sure you have a copy of your passport and passport photo. You will also need a copy of your stamp in your passport and a copy of the little piece of paper you got on arrival. Pay 1900 BAHT and done.

Okay, lets get you back on track terminology wise

First if you showed up here without having BOUGHT a thai visa from a consulate before you got here and were stamped in for FREE for 30 days you did NOT get in on a visa on arrival :O You got in on waht's called a 30 day visa exempt entry stamp

You can go to the immigration office and extend that ONE TIME for 30 more days for 1900baht. THEN you have to leave the country and either come back in on a NEW 30 day visa exempt entry stamp OR go to a thai consulate in a near by country and BUY a tourist visa for thailand.

Just go to immigration and apply extension of stay for 30 days. Simple. More than that u can not get during normal circumstances without leaving the country. Whether can get another extension under coming extraordinary circumstances remains to be seen

Just go to immigration pay ฿1,900 and get a 30 day extension