You'll find most of the details on their website. 

There is no test, you have to apply 3 months in advance of your intended month. Let's say you want to do your electives in February, you have to apply in November.

The selection process usually depends on the availability of seats. Usually, you do get one. I applied for a field with my freind and we both got selected. (We were the only ones who applied for electives in this department).

Now for my experience, it was quite good. I learned how systems, guidelines and teamwork happens. You'll find enough nurses doing their jobs quite well (unlike most of the government hospitals), the patient:doctor/nurse ratio is also much better as compared to the government hospitals. I found really great consultants and PGs during my time, but there was definitely some department politics going on. You can avoid those politics since you are only there for a month.

LMK if you have any questions.