Copy-pasta:
How to idle a specific program:

Do the following:

Click Start
Click My Computer (This PC on Win 8)
Double Click the C: Drive
In the Search bar search for steam-idle.exe
Let it complete the search
Look for steam-idle.exe (It will have the Steam Piston logo)
Right Click it (if more than one use the first listing)
Select Open File Location
Right Click the address bar (directory listing) and select Copy Address As Text

Now Click Start again
In the search bar type CMD
Click the CMD.EXE file in the search results to open a command promot
Type CD and hit the space bar
Right click in the black window
Select Paste
Press Enter

Type steam-idle.exe #####
Replace ###### with the games appid

To get the games appid go to the games store page and it will be listed in the URL