A Kolsch has starting gravity of 1.040 to 1.046, IBUs of 20-30, and SRM of 3.5 to 5. Low hop flavor and aroma. Light to medium body. Slightly dry, winy palate.
Malts can be U.S. or continental, including a fraction of wheat malt if desired.
Hopping should be continental noble hops. The yeast is the tricky part, as to my knowledge there is no available Kolsch yeast. The Goose Island Brewery in Chicago brews a Kolsch using a Kolsch yeast from Germany. This yeast is suggested by Fred Eckhardt. I've used the yeast from time to time and think it's a great yeast, so use this in preference to any generic ale yeast.