How would I replace just the lower ball joint on a 2002 Chevy Malibu

First of all . Make sure that the control arm bushings are good on the control arm. If they have any deterioration or cracking, it would be advised to replace the control arm assembly as it comes with a new ball joint. The price on the control arm assembly is about 55 dollars and the ball joint by itself is about 15 dollars.
The problem with just doing the ball joint is that it is riveted into the control arm and considerable time and special tools may be needed to remove the ball joint from the arm, and possible damage to the control arm can occur, The better process is to replace the control arm assembly.
Make sure to have alignment performed after replacing the parts.

I list a lower ball joint available to purchase.
So you would have to disconnect the lower control arm from the spindle to replace the joint.