It's a time-consuming process, but it isn't difficult. You'll need a tube of high-temp waterproof marine grease... I get the stuff from Yamaha to be sure it's the right stuff. You can redo your clutch three times with one tube, which costs about $7.
Remove the primary clutch and put it on a workbench. remove the philips screws holding the "lid" (for lack of a better term) on. There are seven screws I think.
Carefully pry the lid off, taking care not to damage the big rubber O-ring that helps seal it in place.
With the lid off you'll see a bunch of grease, probably a lot of dirt and dust, and eight round things.
Remove the round things (rollers) and set them aside, on a paper towl or something. Remove the small U-shaped spacers and sliders (I think there are two of each) and also set aside. Remove that O-ring if you haven't already.
Basically you want everything out of the clutch.
Now start wiping all of the grease and crap out using paper towels. When you've gotten as much as you can, soak the clutch in a bucket of gasoline for several hours. It's probably best to check back every 30-60 min and continue cleaning it all out. When I do this, I like to have the clutch completely clean and not leave any built-up grease or dirt behind. This takes patience and time.
You want to do the same thing to the rollers and sliders. However, the rollers have a plastic sleeve and the gasoline may damage them so I don't soak them; I wipe them off and use Simple Green or another gentle solvent on them.
Once it's all clean and dry, I generally put approx one tablespoon of grease in each of the eight "slots" where the rollers sit. Then I put the rollers back in. Then I put the sliders/spacers back in, and the o-ring back on.
Next I put the lid back on and put the screws back in.
Assuming I haven't missed anything, that's it. Note that you need to be sure that the rollers stay in place when you pick the clutch up and put it back on the quad. Also be VERY sure that you get the clutch well seated on the splines of the input shaft or you're in for an expensive surprise (don't ask me how I know this).