Instructions
Go to the entrance to Route 11 on your map. Here you will find a cave entrance. Enter this cave and follow the path here to exit the other side. When you exit, you will find Professor Oak. Talk to him and he will give you the "Flash HM" item as long as you have at least 15 Pokemon in your collection.
Enter the Pause menu when you get the "Flash HM" and select the "Items" menu. Select the "Flash HM" from the list. You will now be able to select an electric type Pokemon to whom to teach the "Flash" ability. Select a Pokemon from your list and they will now have the ability, which can be used in battle and in the Rock Tunnel area.
Go to the entrance to the Rock Tunnel, which is located on Route 10 on your map. Before you enter this area, go to the nearest town and make sure that the Pokemon who was taught "Flash" is in your list of available Pokemon. Enter the tunnel and you will notice that it is extremely dark.
Enter the Pause menu and move to the "Pokemon" option. Move down to select the Pokemon to whom you taught the ability. Press the "A" button and an options menu will appear including the "Flash" option. Choose this ability and the area will be illuminated as long as you have this Pokemon with you.