Use design software, such as Adobe InDesign, or a pencil and some paper to establish a consistent grid that you can set your images and words into. With the average portfolio sitting somewhere between 20 and 40 pages, it is important to establish a unifying grid structure to avoid clutter and confusion throughout.
Determine which projects you feel best demonstrate your strengths, and cut those that you consider to be the weakest. Organize the projects that you wish to display into a list.
Arrange the projects from first to last in a way that creates a narrative of your design process and your own evolution as a designer. Starting with particularly strong work will make a good impression, as will closing with a strong piece.
Place the images and captions from each project in a way that brings the viewer of your portfolio through the various stages and points of each project. Begin each new project's section with a title page that briefly explains the goal, your process, and your end result. Be sure to include all different kinds of media, including the occasional sketches and mock-ups of the design as it evolved.