flight first to China and apply in Thailand

I want my Thai girlfriend to come visit me here in China. I'm a UK citizen on a 2 year contract with a Chinese company. I know I have to send an invitation letter and copy of visa, passport, salary statement, bank account.

But do I need to book the flight in advance? or should I wait until she has visited the Chinese embassy in Bangkok with the paperwork? In other words is a return flight ticket essential for getting it?