Get approval from the rental company if the car is rented! This is very important because they may have decorations that the company provides or have they may have rules and regulations about what can and cannot be used on the car.
It is usually the duty of the Best Man and/or members of the Bridal Party to put this all together. The decorating usually takes place quickly and secretly before the couple leaves the location of the ceremony or before they leave the reception.
Although this decorating can be loads of fun, remember some of the safety concerns as well. Cans are always a classic idea to have clanging behind the car, but these cans can also cause sparks and no one wants that kind of a problem on their wedding day. It is also against the law in some places because of the fire hazards. Also, don't put any kind of sugar products on the car such as whip cream, the sugar will ruin the paint job. One last tip - don't obstruct the drivers view! Window paint can be a lot of fun, but keep in mind that the driver still needs to see where they are going, so don't get too carried away and color in the whole back window.
Now go have tons of fun surprising the newlyweds, and have fun decorating, this is something they will remember for the rest of their lives!
And don't forget... Take lots of pictures!