Can't say much about techniques but I can give insight on what ladies might feel towards cold approaches:
- comments about physical appearance from a stranger might be risky... at worst, you're scary/creepy.
- avoid approaching someone who may not feel safe at the time, e.g. someone walking alone + quiet place... Don't give them a reason to run away from you

Honestly for you as a student, I think it's better to try your luck in university. Join events and clubs, work in committees, you'll learn something plus opportunity to hang out with girls and catch a glimpse of their personalities too. Unless you're that eager to fuck around, then I guess it's just a numbers game -- get rejected enough and you'll grow immune and be able to approach anyone.