The Gameshark codes are:
010344D0
015045D0
010343D0
Note that these are currently only Pokemon Gold and Silver codes, but we'll try to find some for Pokemon Crystal.
To get there, you just enter whatever door is near you with the codes on. You'll be in the Safari Zone's entry room, so you got to turn off the codes.
Now, you could just go back outside and you will find yourself inside the wall of the closed-down Safari Zone building in Fuchsia City, Kanto.
You can get back inside by turning back on the codes and entering a building.
But the purpose of this glitch is to get into Safari Zone. So you go through the north doors in the room with the codes off, and you don't have to pay any invisible man or anything like that.
You'll find yourself in a somewhat-glitched Safari Zone, but most of the tiles are completed. You can't go back in though, and you can go around the glitch tiles you came out from.
If you go up and explore it, you will find some normal grass and a lot of tall grass, and a pond with no sand around it. Unfortunately, Safari Zone is pretty small and there are no Wild Pokemon, not even ?????.
But this shows us that the programmers originally intended to add the Safari Zone, but replaced it with National Park.