How to install a Steam game through copying files from a friend the easy way.

So this tutorial covers a very simple and easy way to transfer game files between different computer on Steam. No "discovering existing files" or "updating".
This tutorial correct as of 27/09/15 when it was written and if Steam changes it's system for games in the future and the folders look nothing like this then disregard this tutorial. You are on your own.

STEP 1:

First things first you need make sure the game is up-to-date on the computer that you are copying from to copy two things: the relevant .ACF file, and the Game Folder.

You will need to find your Steam folder which is by default in C:\Program Files (x86)\Steam but if installed somewhere else I think you would know where it is.

The .acf file you can find in the \steamapps folder(this can either be found in the "SteamLibrary" or the "Steam" application folder)

Now this just looks like a bunch of numbers but each "appmanifest_xxxx.acf" is a file that tells Steam which game are installed, the size of the installation files, and other things. Each file if for one game and the number is the AppID number of the game. 

IT IS IMPORTANT YOU COPY THE .ACF FILE AND GAME FOLDER AT THE SAME TIME as the .acf file only is correct if the game files haven't updated.

One way to find which one you need to copy is to simply open each one in Notepad until you find the right one but there is another way if you have many games...

There may be an easier way to do this but this is the way I have always done it.

Go onto the Steam Store through a web browser(because steam took away the address bar) and go to the store page of the game you want to copy.

The AppID is the number in the address bar. For example for "NBA 2K16" the AppID is "370240" so you would copy "appmanifest_370240.acf".

The next thing you need is the up-to-date game files.
this can be found in \steamapps\common

STEP 2:

Copy the files into the Steam folder that doesn't have the game installed. The folders should have the same layout so put the .acf file into /steamapps and the game folder into /steamapps/common.

Just copy the files into the same folder you took them from.

DO NOT INSTALL THE GAME THROUGH STEAM

STEP 3:

Restart Steam(or open Steam if not already open)

The game should now be installed(white font) and all you need to do is click play to run the first time setup.

DONE!