There is no proper way to spell this. I happen to like the spelling catsup but ketchup and catchup are just as valid.
I first became interested in the origin and proper spelling of this common condiment when part of a discussion board where the question of how it is spelled came up. Trying to be helpful, I surfed the web in order to learn of its origin.
Taken from the Malay word for "taste", which was variously spelled kechap or ketsiap, the original form was made with pickled fish and brine and sometimes other handy ingredients. I suspect that it was much like old-world stews and modern day casseroles in that people would use whatever they needed to use before it went bad. But I could be mistaken.
It was through the importation of the tomato from South America to Europe that what Heinz has become famous for marketing as Ketchup was born. The modern ingredients (as taken from the back of the Heinz all natural Tomato Kechup bottle): tomato concentrate made from red ripe tomatoes, distilled vinegar, high fructose corn syrup, corn syrup, salt, onion powder, spice, natural flavoring.