Hmm...this'll be a bit tricky. Ok... um...
1. go to Cerulean Cave and beat Mewtwo (don't catch him. beat him)
2. do the missingno. glitch (i forgot the glitch, so you tell me :P)
3. catch missingno. and name it Mew
4. Hmm...still thinking...
5. defeat the Elite Four and your rival
6. go to "Pokemon" (in menu) and look at missingno. It's gone.
7. do the missingno. glitch again. (still forgot what it was :P) Instead of missingno, it will be a Nidoran(boy).
8. catch Nidoran(boy) and go to menu ("Pokemon"). It looks like missingno.
9. go into battle with missingno. It's name is "Mew", has the cry of Mew, it looks like missingno., and has Nidoran's attacks.
10. defeat whoever you go into battle against.
11. go back into "Pokemon" (in menu). Your morphed missingno./Nidoran/Mew is a full-fledged Mew! It is a level 50 and has Fly, Transform, Pound, and Surf/Sucker punch (if your character is a boy, it will be Sucker punch. If your character is a girl, it will be Surf. Also, since i said "girl", this works on Fire Red/Leaf Green)
Enjoy Mew!!!
How do you put up a dot border? I'll just make this the border.
My Mew: Adorable
Me: Ready to battle.
Both of us: Unstoppable!