For my masters application, graduate colleges like USC require?
It should be sealed and sent by the University is most North american schools preference. Occasionally they will let you send a sealed envelope with the referees signature written over the envelope seal. However your best bet is to contact the admission department of the schools you want to apply to and ask. This is too important a thing to screw up for such a minor piece of work. The paperwork is a pain, but if its not done properly you may be kissing a great education away:)