My truck, a 1997 Ford F250 Heavy duty truck; 5.8 liter-engine (gas), has been idling high and I need to ask "How do I adjust the idle?"

See if this works:

On the throttle cable, loosen the outside nut until the desired rpm is reached; then, tighten the inside locknut against the throttle bracket. Adjust idle, when the engine is at normal operating temperature.