How does 90 days reporting work in Thailand if I stay with my Thai girlfriend and have no TM 30 or rental contract?

The 90 days reporting in Thailand requires a TM30 form, which must be registered by the landlord or property owner. If you are staying with your Thai girlfriend, she must fill out the TM30 if she owns the property. If it's a rental, she should ask the landlord to do it. There is no rental payment requirement for registration, and as an alternative, spending a night in a hotel could also satisfy the requirements.