Note that a glitchy Pokémon you get from the glitch may sometimes (but not always) freeze the game when its picture is viewed in the Hall of Fame, meaning warping to the Hall of Fame (HP = 118) is not recommend. I don't know if viewing its picture this way can erase the save. In the original Red/Green, encountering normal Missingno. does tend to do that but in Pocket Monsters Blue viewing the picture of this glitch Pokémon didn't.
Don't save and reset the game after activating the glitch, otherwise the tiles surrounding the map will change from blackness to an ordinary tile, so stepping out will put you out of bounds and freeze the game.
1) At least 13 items
2) A Pokémon (Pokémon 2) with a Special stat of 7 and a second move with PP greater than or equal to 4.
3) A Nidoran (Pokémon 3) with a current HP stat equating to the index number of the map you want to warp to. Note that this stat should be lower than 256.
If you don't meet requirements 2 and/or 3, both sides of the exit mat will remain 'blocked' after doing step 3.
1) Go to the Name Rater's house in Lavender Town. Alternatively, you can go to the Daycare man on Route 5.
2) Highlight the 12th item with select, exit with B,B, talk to the Name Rater and choose the second Pokémon (that has a Special stat of 7 and at least 4 remaining PP on the second move) to swap it with a glitch Pokémon. The exit mats should be blocked.
3) Highlight the 13th item with select, exit with B,B, talk to the Name Rater and choose the third Pokémon (the Nidoran) to swap it with a glitch Pokémon. The left of the exit mat should be blocked, but going through the right exit map should take you to a location corresponding to your Nidoran's current HP.