start it up, warm it a bit
then raise the idle speed up to about 1800 rpms by turning the central screw in (with the spring on it)
now turn the flat head screw up near the fuel inlet where the hose joins the carby
screw it out a 1/4 turn, listen to the way the engine runs, you want to adjust it so you get the highest/smoothest rpms
try turning it out another 1/4, if the revs drop then turn it back in again
you can fine tune it again after 10 seconds or so running, it needs to settle after each adjustment.
once you are happy, turn the central idle speed screw back out until the revs are back to normal idle speed.
give it a few blips revs and see how it sounds.
now take the bike for a ride and see how it goes.
if you need to fine tune it now, just go a slight turn each time (1/16th) and try it again