How to use them! that your question? I have been asking that for a long time too, but I know how to do it now, but keep your original copy as standard since you cannot revert back!

1: Copy your selected app to your desktop (Icon folder recommended to store themed app with the icon themes)
2: copy your image (only the part you want)
3: right click your app and click Get info
4: click the app icon at the top and press cmd and the V at the same time or click edit and paste
5: the app should change icon!

Hope I helped! but remember, keep a extra copy, because you cannot revert back once you change the icon!

how do you use these ?

apple products are very good compare to microsoft