Instructions
Withdraw an Electric-type Pokemon that knows the move Flash, such as a Pikachu or a Raichu. Because Rock Tunnel is so dark, you will need Flash to light up your environment to know where you are going.
Head east through Route 9 from Cerulean City. No real obstacles exist here, though there are nine trainers you may have to fight on your way, depending on whether you can get around them without stepping in front of them. If necessary, you can return to Cerulean City to heal your Pokemon and back to Route 9 without worry of fighting the same trainer again.
Enter Rock Tunnel, and have your Electric-type Pokemon use Flash. Walk as far south as you can, then east, then south, then east, and then north. Walk onto the ladder you come across to climb it into the second part of the tunnel.
Walk south, then west, north, west, north, east, north, east, north, east, north, west, and then north. Walk onto the ladder to enter the third part of the tunnel.
Walk south, then west, then north, and then west. Walk onto the ladder to enter the final area of the tunnel.
Walk east, south, west, south, east, north, east, south, and then east, exiting the tunnel at the opening.
Continue south through Route 10 into Lavender Town. Again, there are trainers scattered throughout this route, but you do not need to fight them if you can avoid stepping in front of them.