How do i replace the catalytic converters on 2002 mercury sable?

i have taken the car to several mechanics and they want between 350 to 600 to fix, not including parts. needless to say, i don't have that kinda mu la. i am mechanically inclined and and have fixed different things on different cars. i think i can do it myself. am i in over my head or does it just take a little patients?