This is a phrasal verb, so the stress is on the preposition. The first sound, /ʧ/, is made with the middle of the tongue raised to the top of your mouth, and plenty of contact. It’s like a sneeze, or a steam train.
This means “relax”, or “calm down”, but again, be careful; in the imperative it’s actually surprisingly strong, and could cause offence. Still, it’s what I plan to do this weekend.