If you search, you'll find a LOT of threads on this, but here's the basics.
The Easy Lube system comes from the factory with just the wheel bearings greased. That means if you wish to use it, you have to fill up the space between the two set of bearings, which can amount to half a tube of grease. As suggested, raise the tire off the ground and slowly rotate it while pumping in grease and don't use a pneumatic or electric grease gun. Many have reported failed inner seals with this system and many here advocate doing the chore by hand. To do so, you need to raise the wheel off the ground and remove the wheel, drum and hub. You can remove them together, but I don't recommend it (more on this later). Clean the bearings either with a safe solvent or using compressed air, then repack them by forcing the grease into the bearing race by hand. Whenever you remove the hub, it is wise to replace the rear, aka inner, seal. If you are careful and feel lucky, you can reuse the old one, but doing so risks damaging it and causing it to leak grease when it gets warm, all over you brake shoes, which is not a good thing. For that reason, I do not recommend removing the wheel, drum and hub together, because putting them back on that way is an easy way to screw up even a new seal and cause problems. It goes without saying that this is a good time to check and adjust your brakes, if needed, or do other maintenance. It's also a good time to check your U-bolts for the proper torque, if you've never done it, and grease the shackle bolts if you have the wet bolt kit installed.