i would like to know how to change the rear spark plugs on a 96 Mercury sable v6

Unscrew the old ones, screw in the new ones.

Its tricky, and very easy to cross thread since the plug holes are ''blind''
what l usually do with the plug socket + 10 inch extension is rotate it backwards(counter clockwise) until you feel it click (practice with a coke bottle and its lid to know what l am talking about) the you can screw it in. then put the ratchet on for the remainder/torque