At Don Muang airport, the re-entry permit/overstay fine counter is located behind the passport control, after you get the exit stamp and before the security screening (opposite order from Suvarnabhumi).
You tell to the immigration officer you need a re-entry permit. They will give you a document if you need. Follow the directions, go to the re-entry permit counter in the small room. You don't need anything but you might be charged 200 thb extra as a "service fee" for the photo and filling out the form, or you bring everything including the filled form.
It's open
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this is general information for re-entry permit.