With your completed image selected, click on the "Current Color Swatch", "Custom Palette" tab, "New" button.

Type a name of your choice in the "Palette name:" box. I usually name it the number of colors I'm going to be using, but you can name it anything you want. Next, in the "Palette size:" box type in the number of colors you want to create this palette with. It can be as low as 2 or as high as 216. This is where experience helps. Start out with 32, just to pick a number. Then see what the results are (more on this later).

Now the "Dither by:" box. If your image has a drop shadow, select "Error Diffusion." Otherwise select "Solid." Click "OK".

Please note that the actual number of colors you use will depend on the image you are working on. I tend to start with 32 and see what happens. Trial and error writes the rules here. For example, the above image has a palette of just 23 colors after being put through Ulead's Smart Saver.