The stress is very much on the word “off”. The most interesting thing here is the pronunciation of the t at the end. The sound /ʔ/ is a stop made with your vocal folds, right down in your throat, the same place where you make a /h/ sound, not with your tongue. You can practise this by making any vowel sound and stopping and starting.
You’ll be able to feel your vocal folds close and open!
Be careful with this phrase. It shows very strong disagreement, and could sound rude. Use it with friends in very informal situations. It means “Don’t be ridiculous!”, or “You’re completely wrong!”.