sure, it's rather easy. (though pictures would make it even easier) anyway in the top left corner of the app there is a section with 4 'tabs' (in order they go: system Icons, App icons, Dock, primary drive icon.)
under neath that section is a section that looks like the sidebar area of the finder. This is basically your library where you can make and store iContainers of icons. (you can find these in lots of places like Iconfactory)
On the other side of the app you will have 2 sections stacked, the top being the place that shows the apps or icons that can be replaced, below is the contents of the currently selected iContainer on the left.
to change an icon simply drag an icon or PNG from either the finder or the bottom section onto the icon you wish to replace. (a small plus button should appear on the semitransparent form of the new icon to indicate that this will replace the default icon)
after you've changed an icon a blue drop panel will ask if you want to change the icons/ dock. if you select yes, it will ask for you password, change the icons/dock, and then ask you to relaunch the Finder/dock. some system icons will require you to log out before they change.
well that's pretty much it.