How do you repair the under body of an 88 glx?
It kinda rusty and falling apart?

 

Take it to a competent shop that does restorations. This is not something for amateurs. If it's really bad, you might consider using it as a donor car for a better chassis. I speak from over 35 years experience in doing fabrication and restorations. I have done some historically signifcant cars that their owners would not have brought to me if I didn't know what I was doing. Expect to spend a lot of money to do it correctly, there is no easy way to do it properly.