First of all, you unfortunately need to have no save to access this "cheat".
Next, you need to name your character very specifically, for example naming yourself "15" if you want Mew. You will see how this effects it later, I'll do some explaining. Next, add the level of the pokemon you want, add a "0" of there was to be no space. (NOTE: If you want Missingno, make sure to leave at least two blank spaces.)
Also, to make it a little easier, here is the hex for pokemon to add for the name: https://glitchcity.info/wiki/The_Big_HEX_List
Ok, once you have your character, play until you get to the part where the Old Man explains how to catch a pokemon. Right after this, without doing anything else, fly to Cinnabar Island, and surf up and down the east shore. Then, you should be done, and encounter all the pokemon you want.
(ANOTHER NOTE: You can level the pokemon past 100 if you want)
How it works:
When you are interacting with the Old Man, it puts your characters name into the pokemon encounter database to replace it with the Old Man's name. This will not change until you enter the next zone where you can do a battle, which is the important thing in this "cheat". Once you enter Cinnabar island, where you aren't supposed to have battles, so the name data doesn't change, as you surf it pulls from whats on top of the encounter list- which is your name. The last thing I leave you with is that I personally have NEVER tried this, and I found it on a video and decided to leave the info here for others to follow.
Next, you need to name your character very specifically, for example naming yourself "15" if you want Mew. You will see how this effects it later, I'll do some explaining. Next, add the level of the pokemon you want, add a "0" of there was to be no space. (NOTE: If you want Missingno, make sure to leave at least two blank spaces.)
Also, to make it a little easier, here is the hex for pokemon to add for the name: https://glitchcity.info/wiki/The_Big_HEX_List
Ok, once you have your character, play until you get to the part where the Old Man explains how to catch a pokemon. Right after this, without doing anything else, fly to Cinnabar Island, and surf up and down the east shore. Then, you should be done, and encounter all the pokemon you want.
(ANOTHER NOTE: You can level the pokemon past 100 if you want)
How it works:
When you are interacting with the Old Man, it puts your characters name into the pokemon encounter database to replace it with the Old Man's name. This will not change until you enter the next zone where you can do a battle, which is the important thing in this "cheat". Once you enter Cinnabar island, where you aren't supposed to have battles, so the name data doesn't change, as you surf it pulls from whats on top of the encounter list- which is your name. The last thing I leave you with is that I personally have NEVER tried this, and I found it on a video and decided to leave the info here for others to follow.