The first one in at the upper part of the exhaust manifold or approx. there... very easy to find... it looks like a spark plug...
The downstream is also easy to find.. easier if you lift the car.... I guess you could also find it if you had a flashlight and went under the car....
I don't know how else to explain... they are very easy to spot....
P.S. if you got a code for downstream O2 sensor i would first try changing the upstream... it worked for me... 30K miles already and no codes....