Clutch work is actually pretty simple. Most of the problems which plague a clutch are even superficial to the clutch itself, and are centered on the actuating mechanism, either the cable or hydraulic line (or master/slave), clutch lever, or the adjustments they require.
First, the cable clutch. Typically smaller bikes will have a clutch cable. This works just the same way that a brake cable works, you pull on a lever and a cable pulls on the clutch actuator, which pushes a small rod which actuates the clutch. The cable obviously can become corroded and break, or get jammed in there, or stretch, making the clutch lever sloppy. The repair here is replacement. After you replace the cable you will need to adjust it properly. Here is how to do that:
Hydraulic clutches have a couple extra components, and fail in similar ways. They are set up very similarly to brake lines, and have a master and slave cylinder, just like your front brakes. In fact they are so similar that I have actually used a brake master cylinder in an emergency for my clutch when I had an accident far from home. I switched the brake master over to the clutch side, and ran home only using rear brakes to stop the bike. The lever went the wrong way so it made operation a little challenging, but functionally it worked.
So, if you are restoring a bike that has mushy clutch, or doesn't work, or is stuck, just restore it like you would a brake line. Afterwards you must bleed it and you do that again, in the same way, like this:
It is possible that your clutch itself is to blame, either something is damaged in your clutch basket, or your clutch discs are worn. Sometimes too if your clutch is slipping, the discs may be ok but your clutch springs are too compressed and need replacement. Remember too that rarely, a bike may be predisposed toa certain condition, such as with my bike the GV1200, since the bike has a wet clutch, the clutch will slip if I use synthetic motor oil even when the discs are new.
If you feel as though you need to replace the clutch discs in a bike, don't worry. Its far, far easier than in a car. Here's how to do it:
So, that's basically it. Not too much to it. Most bikes we get to restore don't need clutch work, as a clutch is very capable of surviving the life of the bike, if it hasn't been raced or stunted or ridden by a total idiot. Therefore if the clutch is misbehaving it is usually an adjustment in the cable, or failure of the hydraulic system. But rarely you do get that higher mileage bike that needs clutch work, so I hope this helps.