@Cha****
, I believe you'd show that thai income tax form the PND1 (you can get that online too I think) along with three months worth of pay stubs and other documentation.
I'd stop by the immigration office and get the hand out of what's required when using a work permit and a job here as the income for an extension based on marriage.
That way when your current extension is expiring you would know what to give them to apply for the next year's extension of stay.