Abra Mew Trick

This is probably the easiest of the tricks. First you must have a Abra that
knows Teleport. Then walk to the top of the screen and walk down and
immediately press START then Teleport to the Pokemon Centre in Cerulean City.
Now battle the YOUNGSTER with a Lv17 Slowpoke then talk to the person who
says 'I saw your feet in the grass' then the menu should pop up, then press B
and a Wild Mew will appear.