You need a special tool to remove the retainer ring. I also suggest to get a good repair manual before you start this job. If you have these it shouldn't be hard.
You can find great repair manuals with step-by-step instructions, diagrams, parts locations and other important information here: http://bit.ly/mitchell_repair_manuals_online
Or here: http://bit.ly/AllData_Repair_Manuals_Online