1. The camp in Sin City, Grand Theft Auto is in the red box on the map, as shown in the following figure:

2. After entering the game, you need to find flying tools to enter the military base, as shown in the following figure:

3. After finding the flying tool, follow the position of the red box in the map, as shown in the following figure:

4. You can enter the military base when you reach the position in the red box above. There are helicopters in the military base, as shown in the following figure:

5. In this way, you can fly the helicopter directly, as shown in the following figure: