Disassembling a constant velocity joint is not difficult if you know how. The first trick is getting the CV joint off the end of the prop shafts. Some joints just pull off while others are held with a snap ring or lock ring which must first be released.
Once the joint is off, it can be disassembled by tilting the inner race to one side. This is done by inserting a dowel or similar tool into the splines of the inner shaft and tilting the race as far as it will go to one side.
This will expose one of the balls which can be popped out of its cage window with a small screwdriver. The inner race is then tilted to opposite side so the next ball can be removed, and so on until all the balls have been removed from their cage windows. The cage can now be rotated sideways allowing it and the inner race to be removed from the housing.
What to look for: nicks, gouges, cracks, spalling, roughness, flaking, etc. on the surface of the balls or tracks in the inner and outer races. The cage windows should also be inspected for dimples, wear or cracks. Each ball should fit snugly in its respective cage window because looseness here is what often causes the clicking or popping noises associated with a worn CV joint.
CV joints are precision fit assemblies, so the balls should be kept in order so they can be reassembled in their same respective positions as before. Each ball and track develop a unique wear pattern as the U-joint ages, so mixing up the balls may change tolerances and create problems that did not exist before the joint was disassembled.
If the CV joint shows no wear or damage, it should be okay to reuse. If it does not pass inspection or is obviously defective, it needs to be replaced. Either way, before the CV joint goes back on the shaft it should be packed with CV joint grease (never ordinary chassis grease!).
Special grease is usually provided with the replacement boot along with instructions on how to pack the joint. About a third of the grease is typically packed into the joint, and the rest is put inside the boot before it goes on to serve as a reservoir for the joint.
There is some debate as to how much the grease actually moves around inside the joint and boot as the vehicle is being driven, but it is there for a purpose so it should be used.