Well it is not as easy as you think. Even though I have bought an aftermarket steering ball joints from
http://www.iautobodyparts.com/mg/ to change, but truly couldn’t succeeded. At last I have gone in garage to do this task, although they have taken a charge but did the work very appropriately. However, if you want to try it on your own then jacked the car up, take out wheels, lubricate the nut from rod ball joint and then fit it. However, it would be better to take help of mechanic.