The first step to building a raised panel cabinet door is to prepare the panel. For a 14" x 18" door, you'll need a 10-5/8" x 14-1/2" panel.
Begin by cutting three pieces of 1x4 or 1x6 to 15" in length. While you can use a circular saw or miter saw to cut these pieces, you could also cut them on your table saw using the miter gauge. (Do not use the fence to make crosscuts on the table saw, as end grain against the fence can cause the stock to bind, which leads to kickbacks.)
With the three pieces cut to length, the next step is to make sure that each of the edges is straight (to ensure good glue joints). This can be accomplished with a jointer if you have one, or you can accomplish the same results with a table saw jointer jig. With either tool, take off only enough stock on each edge to make sure that the edge is flat, even and square to the face of the board.