we will be in Bangkok and last time we went, we had to book an appointment first. So confusing at it's not an option for that now. So wondering if they changed something about the booking system or just show up, sorry for not adding that in the post.
You and gf go to immigration, with the owner of the house don't forget the bluebook! A signed copy of her or the owner ID card and a copy and signed copy of your passport then fill some papers then you get tm30 has to be done 24h after you landed as Brandon says.
Side note if you have applying visa before arrival your gf needs to write an invite to the house.