1 - Find your GTA V installation folder (e.g. C:Program Files (x86)SteamSteamAppscommonGrand Theft Auto V). It should have files such as GTAV.exe and bink2w64.dll in it.
#2 - Extract fov.txt and dsound.dll into that folder.
Change the contents of fov.txt to your desired vertical field of view. The default I've set is 68, which translates to about 100 degrees horizontal on a 16:9 display.
#3 - PROFIT! Launch the game!