Might be easiest to go to a hospital and get a written treatment / diagnosis. They ask for quite a lot. I did it all in Thailand at Chiang Mai Ram but Samitivej in BKK was helpful too
I always appreciate the female officers I feel they respond to smiles and friendliness better. Just went in 2 weeks ago and female officer was quick and friendly. Said I'm only need two weeks this time and she said don't worry i'll always stamp you full 6 months (cause of DTV).