It’s like a one car train
Step 1.
Get on the bus via ticket or any other means (smuggle a ranged weapon like a revolver on too)
Step 2. Once your sure you’re in a less public area (like a neighborhood or backroad) pull out your gun and likely a bag
Step 3. After robbing everyone get off the bus where your driver who was following the bus will pull up pick you up. And drive away.