Banqiao

To perform a 'ban-gyo' (mutual teaching/exchange), you need to have a 'Sparkling Charm' or become a 'Forever Friend' to become a 'Sparkling Friend'.

As the commenter above said, it's common to maintain at least a 'Forever Friend' status with your friends and interact daily. Also, the glitter charm obtainable during specific events is another method.

If you trade Pokémon from 2020, they are guaranteed to become shiny!