The script adds working garages to all police stations. You need to stand on the green marker next to the garage and press the F11 button. The police car selection menu will open. Select the required vehicle, it will appear on the blue marker. If you have chosen the wrong car, you can move it to the red marker and press the Z button to remove the car.
For the script to work, you must have Script Hook V, Community Script Hook V .NET and NativeUI installed.
To install, move the Scripts folder from the downloaded archive to the folder with the GTA 5 game.