IconPackager

Choose "Windows default icons" or any Iconpackage You have in IP. Drag and Drop Your new icons to the fitting item-icon and category from explorer (another way is to click "Change" for each and every icon), "Save package as", "Apply" and there You go. Be sure to have permission from the original author if You want to upload Your package anywhere.

The easy (but tedious) method is to launch iconpackager, select the icons and cursors icon on the left bar, and for each of the icon entries in each tab, change that to whatever icon you want to use. When you're done, have IconPackager save that as a package (file-> save package as ...)

You can also manually edit the .iptheme configuration file (it's text). Best way to do that is to open an existing theme file, and modify it for your icons.

Then you can package your theme into an .ip package by using file -> package theme.

HTH

I would like to repeat what c242 said up there.......
Be sure to have permission from the original author(s) if You want to upload Your package anywhere.