First you will need a designer tool to make the design of your icon.
I will choose for adobe illustrator.
When your done with the design just import it to Ps.

After you placed it in Photoshop. Delete the white background layer. Then go
to save as and select save as .TIFF with the options, “Layers” unchecked and
“As A Copy” checked. A TIFF Options dialogue box should appear after hitting
the save button. Itʼs important to have Compression set to “None” and “Save
Transparency” checked. This way, the icon comes out exactly as you
intended.
 

Now we startup Icon Composer. Just drag and drop your .TIFF file in it and it
will resize to the different sizes of Mac icons.
Just take save as and save it. Now you will have a .icns file.

Just open Icns2Rsrc go to file>> open and select your .icns file and save
again into a .rsrc file.

Your icon is finished now.


To insert it into a map just select your .rsrc file and open info (cmd+i).
Click on the icon and copy it (cmd+c). Select the folder you want to put the
icon on. Open the info window and select the icon and paste (cmd+v) your
own icon in it. Just close your info panel and done.

If it does not appear instantly you can restart finder or reboot your mac.
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