Go to the folder that says versions. Copy the folder labeled 1.6.2 and paste it in the same spot, just rename it to 1.6.2Modded (or whatever you want.) Then, go inside the 1.6.2Modded and rename both files in there to 1.6.2Modded (including the json file.) Go inside the json file with notepad (or something similar) and change
Then go inside 1.6.2Modded.jar and mod like you normally would.