http://www.autopartswarehouse.com/details/QQNissanQQSentraQQMoogQQBall_JointQQ19911999QQMOK9449.html
It not that hard at all. You can rent the tool to get them out at Advance or Auto Zone for like 40 bucks...And you do get your money back once you return the tool.
All you do is take off the tire. Un-bolt the lower control arm. Take off the "O" ring snap seal... < in the picture in the link above > And then attach the remover tool. Screw it down on to the old ball joint and it will pop right out. It's shaped like a fork that wraps around the control arm for leverage to press against.
Once the ball joint it out, you put the new one in, attach the boot that comes with the new one. Put the new ring on... screw the grease fitting on abd attach with the new nut. It would also be a good idea to change the cotter pin out. They like to wear out over time. Other than that... fill the new one up with a grease gun and you are good to go.
Alignment is also needed to make sure everything is lined up. Good luck.