Having read all the answers I think I had a unique situation where I had already started downloading the game. Attempts to copy the game still had steam thinking it should download.
So here are a list of steps I think includes the other answers and my situation.
When taking through this, I will refer to stuff as source and target. Target being the pc or files that you want to copy the games to.
Open steam on the target PC.
First we should ensure that the game is not on the target PC due to a started download of it. Right click on the game / properties / Local Files, and if the button "Delete local game content" is enabled, then click it and do that process. The game should still list in your library, just greyed out now.
Close steam on the target PC. Ensure it is not open. (I don't think this is required, but just in case).
From the source pc, either over the network of with usb sticks etc. Copy the game folder from the source pc from a folder similar to this
C:\Program Files (x86)\Steam\SteamApps\common to the similar location on the target pc.
Once the copy has been completed. Open steam on the target pc, right click on the game in your library and click install.