First of all start the application and select the image. By the default Photo Exif Manager opens the Pictures folder. You can change the folder in the tree view at the left bottom of the application. Then in the right bottom corner select the file. After what, it will load image to the preview area and will load exif tags to the tags list.
To add tag click the button "Add Tag" on the Tool Bar or use menu "Exif Tag" -> "Add".
Check tags you need to add and click Ok button.
Added tags becomes highlighted in bold with the default value. You can change this value as in the example: Edit Exif tag. To commit the changes you need save them, use Save button on the Tool Bar or use menu "File" -> "Save". To save image to a different file click the "Save As" button on the Tool Bar or use menu "File" -> "Save As".