[V8] Replacing o2 sensor

[chris brown]

 Hello tonight i changed the fuel filter on my 1990 v8. It was a pain in the ass but it's done. Im pretty sure it was a original filter which would make it over 20 years old. it was bright orange and had the Audi symbol on it. It was a audi part. 

 Tomorrow night i am most likely going to be changing my O2 sensor. It should arrive in mail tomorrow. 
 I don't know where it is, or whats involved. I've got a 1993 4.2 engine in my 1990 Audi. (i swapped out motors) So its not a 1990 engine.
 I could use a little advice on which tools to have handy and a basic over view on how to do it if you fellows don't mind.  Thanks again  chris