How do potassium levels affect cardiac output?

Not directly. Cardiac output is the amount of blood flow generated by the heart over a given time frame. Potassium is a critical electrolyte for body health and cardiac function, especially its rhythm. Although high or low potassium levels can create major, potentially fatal, irregularities in the heart rhythm, those levels do not directly affect the cardiac output per se.
They don't. Unless the potassium gets very very high (>7) or very very low (<2) in which case cardiac output falls to zero (because of cardiac arrest).