From a lifetime of teaching:

Smedley has the basics right.

Always try to start with a "well done" or "I liked xxxx...". In otherwords always find something positive to say about their performance, however poor overall you may have privately thought it.

You then need to identify "areas for improvement" or "things you might think about..." or something similar. In other words say how they might do something better rather than say it was poor, but as said earlier, don't be scared to point out faults. The key is to put it in terms of how they might improve.

If you know the person well, you can get away with statements like "That wasn't very good, was it?" but it can destroy someone you don't know well and who is not very confident.

I like the idea of "How do you think it went?" and quite often used that approach when discussing oral presentations by students with them.

At the end you could say something along the lines of "If you follow up these suggestions, this has the potential to be a very good performance".

Remember that teaching is a process that involves both the teacher and the learner but the onus has to be firmly on the learner to do what is necessary to achieve their aim. Your role is to provide suggestions on how they might achieve what they want to achieve and to encourage them and praise what is good in their efforts.