I applied online. I interviewed at World Wildlife Fund (New Delhi) in Jan 2019
Interview
It's telephonic. they basically ask you about your experience and judges your communication skill...
they also might take a zoom interview in some cases.
Be prepared for question like
why you want to volunteer
where did you got to know about it etc
Interview questions [1]
Question 1
your day schedule...
nothing is majorly focused on your academic background.
they will take you anyway irrespective of how you perform.
I interviewed at World Wildlife Fund (Kuching, Sarawak)
Interview
It was a straightforward process and was put in touch with the right people relatively quickly. Provided CV and other areas of interest and own personal interests were taken into account.