I am assuming you have Metal Gear Solid 2 PC disc or GOG version installed and have also installed V's Fix and have launched the game once.
You will also need to emulate or use an xinput pad (For DS4 run the pad through DS4Windows, Xbox pads are plug and play).
Controller settings inside V's fix should be set to Xbox regardless of the controller you are using.
Step 1. Open the game install directory. Click into the bin folder, find a file called XinputPlus.ini, right click on the file and open with notepad.
Step 2: Now that we have the xinput file open in notepad, scroll down to [Rapid Fire] and change the following variables:
[RapidFire]
Buttons=A|B|X|Y - These are the buttons that can be used with turbo, if you aren't sure, add this for all of the face buttons. If you want to add any buttons, use the key image above and separate buttons with a | .
Speed=1f - the f is important. This is the speed that should be set.
ToggleONOFF=True - Keep this set to True (It lets you toggle turbo)
ONOFF_Button=LSB - This is the button you will use to activate turbo you can choose any from the list of available buttons, There is an image above showing each button and the corresponding code you should be writing in here.
MaskToggleONOFF=True - Preference, if this is enabled the turbo button you set will not interact with gameplay, if it was set to START for example, the Start button would activate turbo but would not pause the game.
EnableAtStart=True - Preference, this will start you with turbo enabled or disabled at game launch.
Your file should look something like this.
Now that we have turbo enabled you're going to hear the voice of an angel every time you enable/disable rapid fire, if you scroll down the ini file to the [Misc] options and edit the SoundVolume= variable you can change it from default volume to a more reasonable volume.
0.10 (I'd recommend) or to 0 for completely off (not recommended as it's easy to lose track of the turbo status).
Done, that should be you with outstanding turbo.
If you want a quick solution feel free to simply copy/paste my own version in. I activate turbo with the Left Stick Button or L3. This can be changed to any button in the graphic above.
https://pastebin.com/08n3V9iy
Copy/Paste the raw versions relevant sections from the pastebin file (the entire [Rapid Fire] section and the entire [Misc] section) to your default xinput file and save the file from notepad: File > Save.