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Rather than looking for a hotel that might just complete a copy of the form, you need to actually find one licensed by the government such as these international youth hostels in Nanning listed by eChinaCities.com. You then need to actually stay there on the night after application for visa extension.
According to what the Beijing Municipal Public Security Bureau, you must have the Registration Of Temporary Residence, the pink form you receive when you check in to a hotel or hostel that is licensed to host foreigners, and it needs to be for the night after you apply (not the night before).
Then, visit the Public Security Bureau (PSB) Exit and Entry Administration office for the municipality where you will be 'resident' on the night following the submission of the application. The PSB officers will issue you with a return receipt once they accepted your application and make the final decision within 7 days.
Nomadasaurus Alesha and Jarryd shared a detailed and useful description of how they successfully navigated the process in 2015.