If you haven't done this before you will surely make many cuts wrong before you finish. It might be a good idea to buy some of the cheapest molding that you can find and experiment with it before going ahead with the final molding. Having someone who has done it before, to help you get started doing it, would be a good idea too. It's not too bad, once you learn the tricks, but accuracy and measuring/marking correctly, and the right technique will play a big part in the end result.
Also, there are three types of molding, those that you hold flat against the fence and saw base (easiest), and those that you hold at 37 or 52 degrees (if I remember the numbers right). This refers to the slope on the back of the wider moldings like crown molding as these do not fit flat against the material that they are being joined to, so they must be held off of the miter saw's fence by this angle for the saw to make the correct 45 degree cuts. If you have a compound angle miter saw there are tables for setting the correct saw angles to allow these moldings to be cut while lying flat on the miter saw. These angles require near perfect settings to get good results.
Charley