To create an icon for a Mac app, you first need to add an image to IconFly and follow these steps:

1. Select the 'Mac Icons' target on the sidebar.
2. Click the 'Generate' button at the bottom of the IconFly window to generate an icon set with all the required sizes.

Learn more about What I can do if an image has the wrong dimensions?

When the image set is generated, you will see a view with a grid of icon sizes.

After that, you can export the app icon assets. Select the export type at the bottom of the IconFly window.