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Start by taking it seriously. Dress as you would for the real thing, arrive on time and keep your phone away. If you treat it as a casual chat, you will practise being casual, and the real interview will feel very different.
Expect to feel uncomfortable. Good mock interviewers ask the questions you hope they will not: why you have a backlog, what you actually did in the project, what you would do if asked something you do not know. If you feel your face getting warm, that is useful information. You have found a place that needs work, and you have found it where it costs nothing.
Answer as you would in the real interview. Do not stop to ask for the answer. If you are stuck, say what you know and how you would find out, as you would for a real interviewer. How you handle being stuck is part of what you are practising.
When the feedback comes, listen and write it down. The urge to explain or defend yourself is strong: 'I said that because...' It is better to wait. The interviewer's impression is what a real interviewer would also have, and explaining why it was wrong does not change it. You can ask questions to understand: 'Which part sounded unsure?' 'What would a stronger answer have included?'
Separate the feedback into what was clear and what was not. Some points will be about content (you did not know a concept), others about delivery (you spoke very fast, avoided eye contact, used long sentences). Both are worth fixing, and delivery is often easier.
Then turn it into a short list. Two or three things, not ten. 'Say my project in under two minutes.' 'Pause before answering.' 'Revise one topic in fundamentals.' Practise those, and ask a friend to test you on them.
Finally, repeat. A second mock interview a few weeks later, ideally with a different interviewer, shows whether the changes worked. Compare it with your notes from the first. The difference between the two is the clearest sign of progress you can get before a real drive.