(1) Select folder/apps/whatever with an icon you want to use to replace the icon of something else.
(2) Go to Get Info of the -whatever-.
(3) Click on the image (a blue square should encompass the image), then select Copy.
(4) Select the folder or whatever you want that you wish to replace its icon.
(5) Get info on it.
(6) Click on the image and select Paste.
This may not work for custom images, as some may appear very small for whatever reason.