The basic aim of tuning is to get just the right amount of fuel going into the engine by adjusting the flow through the needle valve. Winding the needle out lets more fuel flow through and makes the mixture richer. Winding the needle in does just the opposite of course.

It's much much safer to run an engine too rich than too lean. Too lean means it'll get very hot and could do serious damage. Too rich really just means lower revs and not as much power. The trick of course is knowing if it's running rich or lean (at full throttle). If you can see a fair bit of smoke coming out the muffler then chances are it's rich. To know for sure just open the needle a little more, slowly, and listen to the revs. If the revs drop then it's rich and you're on the safe side. If the revs rise a little at first and then start to drop then you were a little too lean to begin with.

When you know for certain it's a bit rich then you can lean it out slowly and listen to the revs increasing. You'll get to a point where you start to wonder if there's been any change so stop...no leaner because you've now got peak revs or maximum power. But this isn't good to fly with because the engine will go leaner in flight and overheat so wind the needle out to make it run a bit richer and listen for a distinct drop in revs. If you've got a tacho then aim for about a 500 rev drop from its maximum.

This is only for tuning the high speed mixture and only with an engine that's been run in, not a new engine. The main thing to remember here is that rich is good and lean is bad!

It's probably best not to overwhelm you with information at first, just get you into the basics.