Classic/Antique Car Repair: 1978 monte carlo, carburetor, linkage

Question
how do I set idle speed.

Answer
It isn't clear from your question whether you are asking how to adjust it, or what RPM to set it to.

To adjust it, you turn the screw that holds the throttle open a small amount when you are not pushing down on the gas pedal.  If you watch the mechanism at the base of the carburetor while a helper operates the gas pedal, you will see that there is a screw you can turn that moves the stop that the accelerator linkage bumps into when you take your foot off the gas.

If you want to know what to set it to, set it so the engine keeps running when the car is in gear, but not moving (set the parking brake to hold it and put it in drive).  Then set the idle RPM to about 650 - higher if the engine is idling rough (shaking), which it will do if the engine needs a tune-up or perhaps has a more serious problem.

Dick