I stand by what I said upthread. Don't criticize. Instead, give examples from other people. Tell the students to record themselves and listen. Let them do their own criticizing.

There's too much criticism in our culture today. Every columnist has to criticize. There's also a strong force that tells people not to make their own music. "Shut up and buy recordings from the APPROVED performers!" - that's the modern way of life. Thus it's too easy for somebody's 'constructive criticism' to be construed as a message that I'm no good.   

Also anybody who has to ask how to criticize probably lacks the huge range of social and verbal skills to carry off the constructive criticism of strangers.