They have a pickup time for each stop on the bus route. The location is usually a fast food place. Do NOT leave your car there. Try to get a friend to drop you off or get a taxi. Otherwise, your car will probably not be there when you return. They tend to be very efficient, so get there a tad early. 5 minutes late and the bus may be gone. Each bus has a series of stops, as many as a dozen, but usually in a relatively small area en route to the other coast. Once you get past those, it's an express ride to the port. It is usually about 6 hrs, which is not that much more than driving yourself. They usually have a stop half way along at a fast food spot for folks to stretch their legs and grab a bite to eat.
Other than that, it is your basic bus ride. Smelly toilet in the back, and ichy seats...but you can just relax. I often nap enroute.
At the end of the cruise, they load you back up and you do the whole thing in reverse. Let me kinow if there is anything else you want to know.