What documents and steps are needed to change from a work permit to a non-immigrant marriage visa in Thailand?

I will go under the premise that you're not on ANY non-B visa and instead on an "extension of stay" based on employment (look in your passport to see.

First you are not switching from a Non-B to a Non-O visa at all you are only changing the reason for your extension

You cancel your work permit, you get documentation from your employer stating you stop work on xxx date, you go to the immigration office and cancel your current extension and immediately apply for an extension based on marriage by meeting the financial requirements for the marriage extension (400K baht in a thai bank account in your name only for 2 months before you apply). you pay 1900baht.

You'll get a 30 day under consideration stamp and then go back when that runs out to get the year extension inked in.

GO to your immigration office and get the hand out listing the requirements for a yearly extension based on marriage,

so you can work on getting the documentation together.