There's nothing so disheartening as to get to the end of designing a wonderful interface, only to have someone point out that if you resize it, it looks horrible, or even worse, it loses its usability. Once you have a shape diagram laid out on paper, it's time to create the base of the elements, the bottom layer of the shape diagram. If you have a background, place it in the open image as the background. It's very difficult to put it in later. Next, we need to create the main shape.
Rectangular shapes are fairly simple, as are circular ones.
Use the line tool with a starting arrow to create triangles. Combining shapes can result in some really interesting interfaces. In this example, we'll create what is commonly referred to as a 'gun' shape. This refers to a vertical 'handle' area, combined with a horizontal 'barrel' area. Go to the channels palette, and click the icon for 'Create New Channel'.