This is the easiest job you will ever do in relation to the "return" from getting it right.
Once you get your new bean-can, install it and adjust the timing so that the bike will start and run reasonably cleanly - do not worry about the ignition being overly advanced or retarded at this time as it will not be staying that way long and will not be under any load.
Leaving the bean-can securing bolts just tight enough so that you can (with some effort) still move the bean-can, connect your timing light and start her up.
Using either the throttle tension screw or an assistant, set the engine speed to 3,500 RPM.
Now, using the timing light look in the timing hole and see what you can see, you may have to move the bean-can a little to find some flywheel marks.
What you are looking for is the mark on your particular model that means "full advance" (on mine it is "Z") it helps if before you start this process you find your flywheel marks and mark them with different coloured paint, but this is entirely optional and frankly depending on how bright your timing light is may simply be waste of time.
Now there are lots of methods to be pedantically "correct" about setting the timing, these involve drilling additional holes, making up "sights" etc. I will express my personal opinion that these are all a waste of time and that providing you set the timing in the right "postcode" your bike will run just fine.
Ok so now we have a nice stable full advance mark right in the middle of the timing hole Yay. You might think that your job is done, but it isn't, temporarily nip up the bean-can bolts and release the throttle tension or have your assistant allow the engine to return to idle.
First problem may be that it is now idling at 2,000 rpm, or perhaps at 600 rpm - in any case, not where you want it. Adjust your idle using the usual methods and then wave your timing light in the hole again - you are looking for whatever passes as the idle timing mark on your model. If you can see the mark at all I would call "job done", nip the bean-can bolts up, put the rest of the bike's clothes on and go riding.
BUT, if you cannot see the idle mark you need to work out if your idle timing is advanced or retarded, but before you do that stop the engine and then restart it, a retarded engine will tell you by being a bugger to start.
The next bit is a decision you have to make, depending on where your idle timing is (remember we set the full advance timing), e.g. if your idle timing is overly advanced, you might want to compromise your full advance timing a little.
OTOH, if your idle timing is retarded, you probably need to change your crank timing gear and timing chain real soon now.
Lastly, something I skipped over, if you see a neat double image of your timing mark - sorry but it is timing chain time and given that few of us are lucky enough to know the service history of our bikes, I'd be changing the crank timing gear too.
Others can chime in from here.
