Repeat until you are satisfied with the way the colors look.
Export the file as a GIF (GIF89A).
If you want to make only the background transparent, check to see if the background color is also used somewhere in the foreground. If it is, you will need to change it to one that is not.
Select the color you want to make transparent and index it as transparent. (The dialog box that appears may give you the option to do this. If it does not, click Options to find it.)
NOTE: If you make a white background transparent and place it on a dark web page, the background may have white artifacts around its border. The same thing may occur if you make a dark background transparent and place it on a light web page. However, this is only a potential problem for GIFs with sloped or rounded edges (diamonds, circles, etc.), not GIFs with horizontal or vertical edges (rectangles). Before you save the changes you've made to your image, view it to make sure you like the way it looks.