When dining in a restaurant, do not apply ketchup directly to your sandwich or fries. Instead, pour a small amount on the edge of your plate and use a knife to spread it on your sandwich.
The same rule applies to fries. After pouring the ketchup on the edge of the plate, you may dip your fries individually. (See the July 21, 2007, posting on “How To Eat French Fries”.)