Always do one at a time and be patient.
So to start remove cam cover & spark plugs plus the timing/points cover.
Adjust the cams using a 17mm spanner on the advance/retard nut until the cam lobe is pointing away from the shim/follower.
Now check the gap using a feeler gauge - i tend to use 0.05, 0.1 and 0.15mm to see how big the gap is. If 0.1 fits and 0.15 doesn't then i am happy. We'll get to what happens if there is no clearance later.
If the gap is incorrect you will now need to remove the shim. You'll need the following tools - a magnet on a stick, a pick and the kawasaki shim tool.
Rotate the cam until the lobe has opened the valve fully. Now insert the tool as per the photo. this holds the bucket open while you then rotate the cam away. This enables you to remove the shim using the pick/magnet.
Note before you do this set the bucket so the little notch is visible - this will make it easier to remove the shim with the pick.
now you've got the shim out you need to measure it. it may be stamped but some are not. If not use a micrometer to check thickness.
Now if you know the gap you have and the shim thickness you can work out what size shim you need. I tend to check all, write them down and work out what i can shuffle around and what needs replacing.
once the replacement shim is obtained then replace it, make sure its seated properly and rotate until the cam lobe is opening the valve. you can now remove the tool and do a full rotation to ensure its all seated properly.
go onto the next one....... easy.
What to do if there is no gap ? Well if there is no measurable gap but you can still rotate the shim bucket then assume it is 0. If you can't rotate the shim bucket then its negative and you'll need to measure again with a smaller shim......
hopefully you'll have measured all and will have a shim smaller from another valve. If the one with negative gap is also the smallest you're really unlucky.......
some useful additional information/pointers from Rich.........
For aftermarket cams Web, Andrews, Kent etc check what clearance you should have with maker, mine run .10 to .20 mm also Mk2s run a slightly bigger clearance than 900s etc.
Write the shim sizes in the back of your workshop manual, then when the next time comes you don't have to take the shim out to measure but can work out if you have the correct ones already.
An offset ring / socket and tommy bar is better as it clears the pickups better.
and you can't emphasise this enough:
Take care when rotating the cam with the bucket holder tool in place, heads have been ruined by rotating the wrong way.
then measured the old shim and found it was different 
