You can buy a special lubricator tool like this one, but I'm sort of cheap and I've seen people using those with a fair amount of mess/spray coming out rather going in the cable. I thought about going to the drugstore to see if I could buy a syringe with a long needle, but wasn't sure if that would work. I ended up using a ziplock baggie with one corner cut open and taped with electrical tape around the end of the cable.
Put enough of your favorite cable lube in the corner of the baggie to cover the cable where it enters the sheath. I used a combination of 3 in 1 oil, 20-50 Mobil One, Autozone general purpose spray lube and some extra virgin olive oil (just kidding about the last one). I wanted something that was thin enough to enter the cable and not drag then it gets cold - but with a little more lubricating ability than WD40. I think any lubricating oil that isn't too thick when it gets cold would be fine.
It might be obvious but you have to remove the cable from the clutch lever before you tape on the baggie. To do that you loosen the middle cable adjuster, which is in front of the engine, by unlocking the locking nut and screwing the adjuster barrel in (clockwise). Loosening the locking nut takes two 12 MM open end wrenches. Or one 12 MM and one 1/2 inch in my case since I couldn't find two 12 MM wrenches and I have about ten 1/2 inch wrenches.
You also have to loosen the thumb wheel on the clutch lever cable and screw that adjuster in until you have enough slack to release the cable from the clutch lever. There's a groove cut into the threaded portion of the clutch lever adjuster that you line up to free the cable (it's a lot clearer when you look at it). You might need to use a small flat head screwdriver to gently pry the cable end piece from the clutch lever.
The baggie method is generally a gravity feed - but I didn't want to wait around while lube hopefully dripped through the cable. If you capture a little air in the baggie and ziplock it closed, you can carefully squeeze it to force the lube through the cable. Make sure you have something to catch dripping oil underneath the middle adjuster and the point where your clutch cable enters the engine. Squeeze the baggie until the lube comes out the other end and your done.
Pretty much done anyway. You have to put the cable back in the lever and unscrew the middle and upper adjuster until your clutch feels right. Once your happy with the feel of the lever tighten the lock nut on the middle adjuster and the thumb wheel on the clutch lever. I try to set the middle adjuster so the clutch lever adjustment is threaded about halfway in so I can fine tune it either way at the lever by hand and not have to use wrenches to mess with the middle adjuster.
There are some measurements given for lever free play (1/8 inch) but basically you want the clutch to start disengaging with about 1/4 lever pull (you can feel it). You want the clutch to completely disengage when you pull the lever in (so you aren't grinding gears/trying to hold your bike from creeping forward when it's stopped in gear) and completely engage when you release the lever (so the clutch plates don't slip and burn out).
Probably a good idea to do some stops and starts in a non-congested area to make sure the clutch friction zone is where you like it before taking off in traffic. Slow speed riding is really dependent on using the friction zone and small changes can throw you off if you're used to a certain sweet spot.