(first make sure u have fly in a pokemon of ur party
and u must have fought the guy with a slowpoke in north of cerulean city and the gambler outside the underground (lavender side))

Go to Celadon city
Go through the Underground Tunnel
Once u have reach the outside the tunnel save (just in case u do a mistake)
step down once and immediately press start (u must be fast)
then fly to cerulean
the gambler will try to fight but u will fly before
go and fight the guy with a slowpoke
head to lavender and go to west
the menu will appear
get out of the menu and mew lv:7 (i think) will appear (make sure u have bought plenty of Ultra balls or if u have a master ball)



PS: by doing this u can also catch other pokemon all at the same level like nidoqueen, jynx, raichu...