1
The firefighter drove the red truck.
To sign this, you would sign 'FIREFIGHTER', then 'DRIVE', followed by 'RED' and 'TRUCK'. Emphasize the firefighter sign by clearly tapping the back of your hand against your forehead.
2
My sister works as a firefighter.
You can express this by signing 'MY SISTER WORK FIREFIGHTER'. Make sure to use your dominant hand for the firefighter sign to keep your overall signing smooth and natural.
3
Call the firefighters!
Sign 'CALL FIREFIGHTER'. Because this is a command about an emergency, use an urgent, serious facial expression to convey the emotion and context of the situation while signing.