Funnily enough I just did my valves yesterday after 20k kms since the last check. The reason I left it so long is that it is a real pain in the neck, literally. As well as a pain in the back which I will explain shortly.
This is the gauge you need to get...
https://www.ebay.com/itm/Motion-Pro-Tappet-Feeler-Gauge-004-005-10mm-13mm-08-...I bought two of them because it's reckoned to be a good idea to have one gauge inserted while you adjust the other side. However after breaking my back yesterday I realised that it's not worth the trouble to do that.
While it is easy in theory to do the valves, in practice the actual measuring is very very very frustrating. It's easy to get to the valves themselves, just whip off the tank and remove the valve covers, then the nightmare begins because you everything is in the way, even your own hand gets in the way, bolts get in the way and other feeler gauges get in the way.
In the end I decided it's best to not try and have one gauge inserted while adjusting the other side, just do them one at a time. Now the best way I found is not to have a gauge in and try to tighten it down then remove the gauge, that will reduce you to tears.
I found the best way is to measure the gap, you might want some normal flat gauges to determine the gap if it is out of the range of the motion pro, linked to above. Then with a ring spanner you loosen the lock nut and have a guess as to how much you need to turn the screw, tighten the locknut, then check, then loosen the lock nut, turn the screw, tighten and check and keep doing this till you're happy with one side, then do the other side, then check the first side again. This is actually easier than than trying to do it with a feeler gauge in.
Things to watch out for, when taking the vacuum hose off the carburettor, careful not to drop the little clip and lose it. Also when removing the larger breather hose that goes through the top of the right hand chrome cover, also do not drop the wire clip on the end of that. To remove that you'll need to take off the chrome cover push it to one side then undo the breather hose. You have to be aware that there are two little rubber caps that are attached to the chrome cover underneath, if you are not careful they can fall on the floor and roll away and you'll spend and hour looking for them when you come to assemble.
When you remove the speedo cable at the engine you'll find that it unscrews. Pay careful attention to the small seal because that will fall off and you won't remember which way it goes so make sure you look at that, then cover the end with a plastic bag and seal it with a rubber band so you don't drip grease or lose the seal.
Some people find it difficult to finagle the tank off because the petcock gets in the way a bit, but it's nothing to worry about just make sure when you take the tank off that you pay close attention to how you wiggle the tank so the petcock clears the frame, so you can reverse the procedure later.
Do not forget to plug the electrics back in under the tank before you put the tank back because when you're finished and go to start the bike and nothing happens you'll have to take the tank off again. You only do this once at most.