The best way to do it is with an exhaust gas analyser. If you don't have one of those available, then I have found this way works quite well.
1. Start & run engine until warm, and choke is no longer needed.
2 Wind out idle speed adjustment screw until it is no longer touching the throttle mechanism. Slowly wind the screw back in until it just touches the throttle mechanism, then give it 1 1/2 more turns.
3 Wind the mixture adjustment screw all the way in, then wind out 2 turns.
4 Start the engine. It will probably idle at a relatively low speed for now.
5 Slowly, 1/4 of a turn at a time, wind the mixture screw back in until the engine starts to falter. Wind the screw back out slowly until the engine start to run properly again, and then continue to wind the screw out another 1/2 turn.
6 Turn on the headlights to put a little load on the engine, and then wind the idle speed adjustment screw until the engine is idling at about 800 rpm.
7 Turn off headlights, engine speed should increase slightly. Quickly but smoothly, apply the accelerator about 3/4 of the way down, then quickly release the pedal. The engine should accelerate smoothly without misfiring, and slow down with out backfiring, although there may well be some slight popping heard as it slows down.
