The sign we use for bus looks like the motion of turning a big steering wheel back and forth, with your fists making the circular steering-wheel motion on either side of your torso. It is like you are driving a bus.
The bus sign in ASL is usually finger spelled (B-U-S). However, for simplicity, we use this commonly improvised sign when working with young children.
Vehicle signs like bus, car, and truck are interesting to most children. The signs are similar, you just make the steering wheel bigger for the bigger vehicles, and make the steering wheel smaller for the car sign.
If your baby has older siblings who take the bus to school, try to teach this sign contextually. You can otherwise teach it in association with the popular children’s song, “The Wheels on the Bus”.