Sigh. Read the instruction manual. It explains it very clearly. Also, one of the two tutorials that come with the game explain everything and actually SHOW you how to do it. All the answers are right there for you. It's two pages, and you would have gotten your answer a lot faster by just reading the simple manual, or playing the tutorial.
This stupid question gets asked way too often on way too many RCT2 forums by way too many lazy people. Sigh.