madcat75,
The basic procedure is the following:
1 - Start Icon Packager > View menu > Package Builder.
2 - Acquaint yourself with the icons that are available for use in the package (start with the default ones shown, you can add additional file types after you are comfortable with the basics). Close package builder and Icon Packager temporarily.
3 - Make a folder and fill with the icons you want to use for your package (this may be various ones that others have made or that you create yourself).
4 - Once you have all the icons in the folder from step #3, reopen package builder as in step #1.
5 - Open the folder that contains your icons from step #3 (if not still open).
6 - Drag-n-drop the icons from your folder onto the various icons in package builder until you have all the icons you want modified in the builder to your liking.
7 - Save the package using the 'Save' function buttons on the top toolbar of package builder (this will save the package to the IconPackager directory).
8 - Close the package builder and Icon Packager.
9 - Reopen Icon Packager and your new package should now be listed.
10 - Select your new package > click the 'Load' button > click the 'Apply' button to test the package and to modify your system icons.
If all goes well, you are done.
Note - if you intend to upload a package and are using another artist's icons in the package, you must obtain and include permission in the upload (usually a text file inside an uploaded '.zip' file that includes the package and permission as needed).
Good luck. 