1. On your desktop, select the icon you would like to use
2. Select "Get Info" (command-I) from the File Menu for that icon
3. Select the icon in the upper left of the window
4. Select "Copy" (command-C) from the Edit menu
5. Select the window
6. Select the file/folder you want to apply that icon to
7. Select "Get Info" (File menu) for that icon
8. Select the icon in the upper left of the window
9. Select "Paste" (command-V) from the Edit Menu
10. Close the window

and if you want to make one from scratch use this
http://www.macupdate.com/info.php/id/6492