How to replace the O2 sensors?

I have a 1998 Mystique V62.5L, I need to replace the O2 Sensors, My trusted mechanic says that there are four, however my Haynes Repair book only mentions 2( 1 before and 1 after the catalyst). Is this True? Is this something that can be done myself? If so what tools do I require and what do I need to do step by step...My Haynes repair book goes into such poor detail on the proceedure. Thanks for what ever help you guys can provide!

Covered in the Duratec Maintenence on contour.org. Scroll down by the O's (i.e. Oxygen Sensor replacement).

http://www.contour.org/ceg-vb/showthread.php?t=41