Note: Make sure your bike is in neutral when you do this else you might move the bike accidentally!
Underneath the radiator at the front of your motor is an inspection cover:


You can do this job without taking off the radiator splash guard but it will make the job easier to undo the two JIS screws (What is a JIS screw? See this link – and in these photos the screws have been replaced with Allen head bolts) and take off said splash guard:

With a 6″ extension and a 17mm socket we can reach this without removing the radiator splash guard or doing anything too tricky, but it does make it tedious when it comes to reinstalling the cover:

Once that cover is off:

…you can see the bolt that sits at the end of the engine’s crankshaft:

Conveniently, this bolt also takes a 17mm socket. Now, if you put your socket on this bolt and rotate you should be able to turn the engine over. This is a bolt that threads into the end of the crankshaft, so rotate clockwise (tightening motion) lest you loosen the bolt… The engine will be easier to turn over if you remove the spark plugs, as then you won’t be fighting engine compression.
If it’s very difficult to turn or you hit an obstacle at some point don’t force it, investigate why! Also, if the motor is seized I wouldn’t suggest trying to free it by putting an impact wrench on this bolt; you’d risk trashing the threads in either the crank, the crank bolt or both in the process.
If you’re trying to get the motor to stop at a certain point (like when you are checking timing) you may want to use a breaker bar in place of a ratchet wrench so that you have more control.