Restart your game and play until you get the HM02-FLY from the house west of Celadon City.
DO NOT fight the following trainers:
- The YOUNGSTER with Level 17 SLOWPOKE on the road to Bill's house (Route 25).
- The GAMBLER with two POLIWAGS & a POLIWHIRL (all Level 22) between Lavender Town & Saffron City (Route 8).
¤ Make sure you have a POKéMON that knows FLY.
¤ Now on Route 8, you need to stand in the front door of the Underground Path.
¤ SAVE your game.
¤ Walk down one step and immediately press START.
If you succeed the Menu will pop up before the GAMBLER sees you.
¤ Go to your POKéMON List and FLY to Cerulean City.
The GAMBLER will see you just before you fly away with the sign [ ! ] on the top of his head.
Now notice that your START button is no longer in function.
¤ Go north across Nugget Bridge and then head east until you find the YOUNGSTER.
¤ Now fight him and his SLOWPOKE!
After winning the battle with YOUNGSTER your START button will work again.
¤ Then FLY to Lavender Town, and head west into Route 8.
As soon as you enter the narrow path, the Menu will pop up by itself. Now Press B to exit... and a wild Level 7 Mew will appear!