What I've done and how to replicate it:

1. Copy a block of 0x1068 bytes starting from 0x383DE and move it to 0xFC000.

	These are the pokemon's base stats. They're 0x1C long and contain most basic info about each (150) pokemon. They're in pokedex order. You're free to add data after this (after Mew when you move it there in step 2). For more information on what each byte does, go to the bulbapedia article.

2. Copy a block of 0x1C bytes starting from 0x425B and move it to 0xFD068.

	This puts Mew's base stats after Mewtwo's.

3. Change the byte at 0x153B from 0x0E to 0x3F.

	This points the game to the right bank of information to find the basic info.

4. Change the two bytes at 0x1578 from 0xDE43 to 0x0040.

	This points the game to the right exact address to find the basic info.

5. Change the two bytes at 0x1593 from 0x5B42 to 0x6850.

	This points the game to the right exact address to find the basic info for Mew.

6. Change the byte at 0x159C from 0x01 to 0x3F.

	This points the game to the right bank of information to find the basic info for Mew.

7. Copy a block of 0x7C6 bytes starting from 0x1C210 and move it to 0xFF000.

	These are the names of the pokemon. They're stored as exactly 10 character strings. Any extra characters for a name are filled in with 0x50. You're free to edit the MISSINGNO.'s as well as add new ones after. They're in exact order of the internal ID number.

8. Change the byte at 0x2FA3 from 0x07 to 0x3F.

	This points the game to the right bank of information to find the names.

9. Change the two bytes at 0x2FAE from 0x1E42 to 0x0070.

	This points the game to the right exact address to find the names.

10. Copy a block of 0xBE bytes starting from 0x41024 and move it to 0x42800.

	This is how the game determines the Pokemon Number of each pokemon based on their internal ID. You'll notice it's sprinkled with 0s for MISSINGNO. You're free to edit these as well as add new ones at the end.

11. Change the two bytes at 0x41017 from 0x2450 to 0x0068. Do the same for 0x41002.

	This points the game to the right exact address to find id to pokemon number conversion.

12. Copy a block of 0x98 bytes starting from 0x725C8 and move it to 0x73BA0.

	This is how it determines which palette each pokemon uses (in SGB mode). In Pokedex Number order. You're free to add more at the end.

13. Change the two bytes at 0x71FB1 from 0xC865 to 0xA07B.

	This points the game to the right exact address to find pokemon palettes.