Flying is a very useful ability in Pokemon games. With its help, you can quickly move to any city or location in which you have already been. This will save you a lot of time when, while traveling through old locations, you need to return to the city. In Fire Red, the Flying ability is hidden and you need the Cut ability to activate it.
Steps

Step 1. Get to the city of Celadon
This is one of the largest cities in the game, and you can only get into it after you defeat the champion of the Vermillion Hall. In order to get to Celadon, you need to follow the underground path on Route 8 and bypass the city of Saffron, which is currently closed.

Step 2. Exit Celadon to the west to get onto Route 16
Until you get Pokéflite, this route will be closed by Snorlax.

Step 3. Use the ability "Cut" on the tree directly above you
As soon as you enter Route 16 and before you reach Snorlax, you will see a tree.
If you do not already have the ability to "Cut", get it from the captain of the ship S. S. Anna, which is moored at the dock of the city of Vermillion. To get on board, you will need a ticket, which you will receive after helping Bill. In order to get into the hall of the city of Vermillion, the ability to "Cut" is necessary

Step 4. Continue west and go through the checkpoint
Enter the small house west of the entrance.

Step 5. Talk to the girl
It will give you the HM02 "Fly" ability.

Step 6. Open your bag and go to the Key Items section
Click on "TM Case" and then select "FLY" from the list. Click on the "Use" button in the menu.

Step 7. Train one of your Pokémon to fly
Choose from your group the Pokémon you want to train with the Flight ability. The Pokémon must be of the flying type, and next to it must be ABLE! You can only train the Pokémon that is currently in your group by flying.
- The Flying ability will occupy one of the ability slots, so pick a Pokémon that has an ability that you no longer need.
- In order to use the Flight ability, a Pokémon with this ability must be in your group. Teach a useful Pokémon that you often carry with you in flight.

Step 8. Use the ability "Flight" to quickly move between cities and locations
The ability "Flight" will allow you to quickly move to any city or location in which you have already been. Having applied the ability "Flight", you will see a map on which you can choose any city or location where you have already been.