What the customer must provide:
A valid form of payment
The vehicle itself in drivable condition
Access to the OBD port and engine bay
Any DMV notice if the registration explicitly says STAR required

What the customer must do beforehand if they want to avoid failure:
Make sure the check engine light is off
Ensure the car has been driven enough to set emissions readiness monitors
Do not disconnect the battery right before the test
Confirm the gas cap is present and seals properly

What the customer does not need:
No paperwork beyond the DMV notice if applicable
No proof of prior smog history
No explanation or justification to the inspector