US /bɛlt ʌp/
・UK /bɛlt ʌp/
If you needed to, you would adjust the seat position, or you can say just "adjust the seat" and "adjust the mirrors." Now I am going to buckle up, or I can also say "fasten my seatbelt," or "put on my seatbelt." You could also hear the phrases "strip in" or "belt up." I'll put the key in the ignition and start the car, or start the engine.
If you needed to, you would adjust the seat position, or you can say just "adjust the seat" and "adjust the mirrors." Now I am going to buckle up, or I can also say "fasten my seatbelt," or "put on my seatbelt." You could also hear the phrases "strip in" or "belt up." I'll put the key in the ignition and start the car, or start the engine.