Here are the steps to create an app icon on Mac OS X:

1. Open the image you want to use for your app icon in Preview.
2. Select the image by clicking on it, and then press Command + C or go to Edit > Copy to copy the image to the clipboard.
3. Open the app you want to create an icon for, and then right-click on it and select “Get Info” or press Command + I.
4. Click on the app icon in the top left corner of the Info window to select it.
5. Press Command + V or go to Edit > Paste to paste the image from the clipboard onto the app icon.
6. The new image should now appear as the app icon. Close the Info window to save the changes.

Note that the image you use for the app icon should be in the PNG format, and should be at least 512×512 pixels in size to ensure it looks good on all devices. Additionally, you may need to restart your Mac or clear the app cache for the new app icon to appear in certain cases.