1. Reach Goldenrod City: This will happen naturally as you progress through the game. You'll need to beat the gym leaders in Ecruteak and Violet City first.
2. Enter the Game Corner: The Game Corner is a building located in Goldenrod City. You can find it by walking south of the Goldenrod Department Store.
3. Collect enough Game Corner Coins: You'll need to play the various games in the Game Corner to earn Game Corner Coins.
4. Redeem Coins for HM02: You can find the HM02 Fly in the Game Corner's prize area. You'll need to spend 100 Game Corner Coins to buy it.
Important Note: Fly is a very useful HM (Hidden Machine), but it's not a guaranteed win. It allows you to travel between already-visited locations, but you still need to defeat the gym leader in any city before you can Fly to it.