Personally I prefer to use a vacuum gauge.
Connect vacuum gauge to the inlet manifold & external tach to the ignition system, then start engine. [allow engine to reach full operating temp first & this is probably best done by going for a drive BEFORE connecting the Vac gauge & tach]

Restrict the airflow in the vacuum gauge tubing by using either a restrictor or by mechanically squeezing the tube [vyce grips?] untill the needle on the gauge stops jumping about. The needle must give a nice steady reading.

Turn MIXTURE either way & watch vacuum gauge. You are looking for the highest available reading.

When highest reading achieved, then adjust idle speed by turning the IDLE SPEED adjustment screw untill correct idle speed is achieved.

Repeat idle MIXTURE adjustment in attempt to improve vacuum reading.
If reading can be improved, then re-adjust idle speed & repeat idle mixture adjustment.

Once the mixture & speed are right on the money the polution cops want you to lean out the mixture by a specific value, often a quarter turn clockwise on the mixture screw but I usually richen up to the tune of a drop of about 20-25 rpm then readjust the idle speed.

Once the idle is correctly adjusted with this method, then a fully serviceable engine will tick over like a swiss watch.