To do it manually, use any mp3 tag editor. Many are freeware.
I have lots of songs each missing some info of the ID3 tag. In some it is the year, in others the album, maybe the album art ... In general too many songs to do anything manually in iTunes.
My guestion is, if there is a way (programe ...) that I can use which will fill in the missing info of the songs so I don't have to do that.