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Find the idle adjustment screw. Start the engine and let it warm to running temperature. Turn the idle adjustment screw to tighten. Listen to the car's engine for sounds of roughness and readjust if necessary. Replace the air filter and finish the job.
There is an idle speed screw that adjust the opening of the throttle plate, and an idle mixture screw that limits the flow of fuel at idle. You want to adjust this second screw. As always, consult the owner's manual or shop guide for your make and model, if you can't find the screws. Just as you did with the fuel/air mixture, let the engine warm up some to make sure you're adjusting at actual running condition. Turn the screw clockwise, no more than a half-turn, and listen to the engine. Most owner's manuals will have an optimum speed to set the idle, though you have some wiggle room if you prefer it higher or lower. Consult you owner's manual for the number and consult the tachometer as you adjust. It should take about 30 seconds for the engine to adjust to the change you made, so don't get happy fingers and over-adjust. Make slow turns and listen closely to the response. When you've got the idle adjusted to the proper specifications, or to your own preferences, kill the engine and replace the air filter to finish the job.