2 Thrust levers (both) . . . . . Retard to mid position
Selecting Alternate mode can occur pretty much anytime during the flight, so the throttles could potentially be anywhere from idle to max climb. Since selecting alternate mode off idle will nearly always result in an increase in N1 - potentially quite sizable - it's a standard instruction to set the throttles to a 'mid' position before selecting Alternate (those words appear for all current production Boeing aircraft).
Obviously, if the throttles are already 'mid' position or lower, there would be no need to retard them.