once the cover is off, be careful not to drop anything inside. if a small bit falls in, it may end up lower inside the engine. if you can't fish it out with a magnet you may have to open the engine completely. if you're nervous, better to put some rags inside to make sure nothing falls in. in the close up view of the cams you can see where the three rubber gaskets A B and C are. the two rubber gaskets B and C look like two small circles joined together (that's where the spark plugs go through). there are two metal bits E and D that hold the two smaller rubber gaskets B and C in position, and these bits can come out and fall in if you're not careful. If you just want to check the clearance you can leave them in but you better know where they are before you work on the valves. if you take them out, put a rag in, first, in case you drop one.
the rubber gaskets you can reuse (till they start leaking, a mechanic told me), although some might say you should change them each time. I have new gaskets but I am reusing the old ones for now. even if the small ones look a little out of shape (mine did), the metal cover will hold them in place anyway. just make sure no grit falls on the gasket as that will prevent them from sealing.
now remove the two covers from A and B with a big flat screwdriver or something like a coin.
If you insert a socket in A and turn counterclockwise, you will see the cams C move. you'll see the lobes of the cams rotate. you want to check the valve clearance when the lobes of the cams for that cylinder point away from each other with the same angle.
the exhaust valves are in front (of the bike), the intake valves in the back, so you want the exhaust valves and intake valves to point away at the same angle. This position (TDC, top dead center) for cylinder 1 is also marked in the view port of B with a line or 1T.
in the position 1T you can check the two ex. valves for cylinder 1 and the two intake valves for cylinder 1. insert a feeler gauge in the gap. Now the tricky part is that the cam pushes onto the bucket below (you can see this as you rotate it). that means that if you push hard enough, you can insert a feeler gauge that is actually thicker of the clearance. So, the actually value of the valve clearance is a little imprecise in that it depends on how good you are at "feeling" the right amount of force to push it in. if it goes in without force, the gap is bigger than your gauge. if it goes in with too much force, the gap is smaller than your gauge. the right measurement for the gap is when the gauge goes in with a little force, but you can slide it off easily.
It's not a big deal if you're not precise to the 0.01 mm, as the gap has a range of correct values, so you just want to check that the gap is within that range.
now if you buy a cheap feeler gauge you'll probably have big intervals between your different gauges. the one I have, for example, had intervals 0.15, 0.20 and 0.25. that's enough for a rough measurement, but not ideal. so what I did was add two gauges, say 0.15+0.06=0.21. That's a little better, but adding two gauges is not ideal either. So now I want to buy a gauge that has all the intervals between 0.15 and 0.25 (0.15, 0.16...). [update. turns out such small intervals between blades are not manufactured, so I guess I'll keep my feeler gauge and use two blades together].
the intervals for the ex. valves, from the manual, are 0.22-0.29
mm. for the intake valves are 0.15-0.24
once you measure all 4 valves for cylinder 1, keep turning till you get to position T2. that's the position to measure the four valves of cylinder 2.
it's easy to measure the valves in front of you for cylinder 1, however the other valves are a bit hidden, and you can barely see the buckets with a flashlight. that's not a big problem, just find the right position by feel and insert the gauge. if you take a good look at the assembly, you'll understand where you have to measure.
in the next figure I'll show a closer view of the cams.