When you drag your PopUp Caption Stack out, you will see the above stack appear on your page.
Now you can drag your desired image into the image drop area. Then drop in the stack you want to use as your caption stack, it can be a plain text stack or any other stack like Joe's Letterpress.
Drag your desired content into the text stack.
Set your desired effect in the settings HUD.
Now preview and enjoy! For an overview of the remaining settings please refer to the PopUp Caption Settings section.