1997 Chevy Astro O2 sensors

Hi there I was wondering how I get at the o2 sensors on bank 1 sensors 1 and 2, are they on the backside of the exhaust manifolds, an also do I have to take the dog house out to do it I have never had one apart, I am a new owner of an astro van and am not looking forward to working on it but can not afford to hire someone to fix it, I am very handy when it comes to working on cars but this van is deffinetly a horse of another colos please any information you cn give me would be a great big helpm thanks alot and look forward to hearing from you.

chad
gonvick, mn