Traditionally it is someone other than the groom or bride that decorates the wedding car. Usually it is the Best Man and his crew so the results are not always that attractive or even that respectful of the human. That is why there is more of a trend nowadays for the happy couple to control how the wedding car is going to look when it drives away from the wedding ceremony.
What the car is going to look like might ultimately depend on how high-brow the car is in the first place. For instance a Jaguar or Bently might not looks so great with “Just Married” scrawled all over it in chalk. In fact you may not want to do much to a car that you are renting for the occasion especially if it means scratching the paint.
You can get wedding car decorating supplies at your local craft store or in the arts and crafts department of a big department store.
If you do want to scrawl “Just Married” into the car then you should buy non-permanent window markers or liquid chalk and write it on the car. Believe it or not Crayola makes a nice brand of washable marker that adults can use on cars.
Be gentle so you do not scratch the paint job while writing on a car. You can also write things like “Just hitched.” Or “Honk. We just got married!”
If you do not want to write on the car then you can buy magnets that have phrases like “Just Married” on them that cling to your car. These are easily removable and do not cause damage.
It is also a good idea to write in soap on the car windows. A big “Just Married” written on the back window has a lot of impact and is easy to clean up later.
It is also a nice idea to buy fake flowers or real ones and ribbon and tie them all over the car. Ribbons, streamers and balloons look great tied anywhere to the car as long as they do not obstruct your vision. The classic place to attach balloons, ribbons and streamers is the antenna of the car.
The most classic touch is to tie some beer cans to some very strong string or rope and hang them from the rear of the bumper. As the car roles away this makes a big clatter. Some wedding supply and party places also sell balls and chains that you can use for the same purpose.