An Edelbrock / AFB style carburetor is pictured, but adjustments on a Holley are very similar
Start by setting the base idle speed screw.
Choke must be completely open and fast idle mechanism completely disengaged
Shown below is a view from the bottom of a carburetor with a properly adjusted base idle screw -
If you are doing this adjustment on the vehicle, you should see about an1/8" of the idle transfer slot above the throttle butterfly plates. If you don't, back off the idle speed screw until you do. If the idle speed screw is adjusted so that you cannot see the idle transfer slots, you will not be able to set the idle correctly on the carburetor because the car will in effectbe running off of the main circuit. This is a very common cause of idle problems.
Idle speed screws are pictured below on an Edelbrock carburetor
Once the idle speed screw has been set, you can set the mixture screws. As a starting point, lightly seat both screws and then turn them out 2-3 turns each equally. Connect a vacuum gauge to a full vacuum source on the intake manifold, start the engine and adjust both screws one at a time an equal amount of turns until you obtain the highest vacuum reading on the gauge.
Your idle should now be correctly set. You may need to touch up theadjustment on the main idle speed screw slightly to your preference.
Normal hot idle speeds are 900-1,250 RPM