Happy to share my experience on this. I have resigned in November and I’m in Bangkok. On last day of my job, I have to go to One stop service center in chamchuro square to cancel my work permit and work visa, at the same time. It took around 2 hours. Officials will stamp on your passport of two weeks’ grace period before you have to leave Thailand. With that stamp, I then have 48 hours to go to Immigration Division I in Chaeng Wattahana to apply for visa extension. I applied for 60-day visa extension and it got approved. The 60-day extension date starts from the end date of the two weeks’s grace period.

Points to note:

-the date you cancelled your work permit and work visa will be the date when two weeks’s grace period starts (better go on your last day)

-officials must see the stamps that you have cancelled work permit and work visa before processing your visa extension

-you could theoretically do the aforementioned steps all in one day (like I did)

-the visa extension that I got was NOT multiple entries

Good luck!