They have four centers -- Beijing, Shanghai, Guangzhou, Chengdu -- and training is at one of them. In Gz, as of four years ago it was offered about twice a year. I don't know if that has changed or might be different at other centers.
There are separate certifications for interviewing and marking the essays. When I did the training in 2004, it was a 4-day course covering both & cost 750 rmb. They now sometimes offer two day courses that cover interviewing only.
All writing is marked at their centers. e.g. for South China, tests are given in Nanning, Haikou, Guangzhou, Shenzhen, Xiamen & Fuzhou, but the writing marking is all in Guangzhou. To get that work you must either live in GZ or be willing to travel there at your own expense each time. I know someone who did that, travelling Zhuhai-Guangzhou. Suzhou-Shanghai would likely work.
You can live anywhere & work as an examiner. In province, they pay for (actually, reimburse about 6 weeks later) bus or train. If you go outside your province, air fare is reimbursed. In my year as an examiner, living in Fujian, I made two out-of-province trips, to Haikou and Nanjing.
Their requirements are native speaker or IELTS 9 (effectively perfect) non-native, (any) degree, TEFL certification (they can be picky about which one), and 3 years teaching experience. There is some flexibility in this if you have some other relevant qualification, like B Ed or a Master's, but I do not know how much. They want three year's experience after the TEFL Cert; I'm not sure if there's flexibility there.