Songs very rarely get started for the first time at games. If you start a song that no-one knows during a match, people don't learn it, your voice just trails off and dies. Songs get learned at groups of fans in pubs and bars - during pre-game meetings, post-game or even in the stadium at half-time. But during a match, the only songs which get sung are the ones a decent group of fans already know the words to. If you want to do spontaneous singing, the core fans must get together to learn the words first.