Instructions
Defeat the first two Johto gym leaders: Falkner, the Violet City Gym Leader, and the Azalea Town Gym Leader named Bugsy. Once you receive the first two gym badges, you will be able to travel to Goldenrod City.
Travel to Goldenrod City by passing through the Ilex Forest and Route 34. Once you arrive, head your party at the Pokemon Center. Purchase Revives, Super Potions and other items at the Goldenrod Department Store. Train your Pokemon until they are at Level 20.
Get the Plain badge by defeating Whitney's party of Pokemon, which consists of Clefairy (Level 17) and Miltank (Level 20). She will not give you the Plain badge at first. Talk to Whitney once she stops crying to receive the third Johto badge.
Go to the Radio Tower and defeat every Rocket Grunt in the building. A Grunt named Petrel will give you the Basement Key upon defeat. Use the Basement Key to open the locked door, which will lead to a room with stairs spiraling downward. Head down the stairs to access the Underground Tunnel.
Battle all the trainers in the Underground Tunnel. Pick up all the red Pokeballs you see on the floor. One Pokeball in the Underground Tunnel will contain the coin case.