How do I change an icon on my Mac?

Answer 1

To change an item's desktop icon, open the "Get Info" window of the item you want to copy, select the icon in the upper left corner and Copy. Open the Info window of the item to change, Select, then Paste onto the same area.

To create your own, Copy & Paste a default icon into a new Photoshop document (so that the size & resolution match). When done designing, Select All the (merged) pixels, then Copy & Paste them onto the selected icon symbol in the Info window.