how to replace ecr and oxygen sensor on my 1992 buick regal custom

What is your question?

I failed my smog test last week and with an expired registration, I'm really pressed for time to get certified. The guy told me I need to replace the ECR and oxygen sensor. How difficult is it to do it myself or do I need a mechanic to do the work? Is there an available source on the procedure and how to replace the components? I found the parts on line that will cost about $40. The complete replacement estimates I got was $180-$275. While that may not be too much for some people, for me it is. If I can do this myself or get the parts and give it to a mechanic to just charge me for the labor, I will be happy. What say you?