What should I do if I'm leaving Thailand one day after my Non-Imm B Visa expires?

Are you currently on the initial 90 day visa or are you on an extension of stay (the stamp in your passport will day extension of stay)?

If you're on the visa then you probably won't even receive a fine since one day is usually waived but may still receive an overstay stamp in your passport.

If you're on an extension of stay then you need to get documentation from your work that states the date you'll stop working, take it to get your work permit cancelled, then take your cancelled work permit and the documentation from your job to the immigration office that issued your extension and cancel that as well.

You should do this before you leave as if you don't cancel your extension (even if it expires) you will not be able to get another long term visa or extension in Thailand again until you do so, and you'll be charged 500 baht per day up to a maximum of 20,000 baht for failing to notify immigration of your change in status.