Restoring a full file by file restore and a differential restore follow the same process, though when restoring the differential backup the overwrite always option should be used. This way even if the file already exists from the full restore, the file will be overwritten when the differential backup is restored.
Here is a link to the UltraBac file by file agent kb, as well as a link that is specific to incremental/differential file by file creation.
The file by file agent kb includes a section regarding the restore process.
Please reply with any questions.