In a straight and level flight the collective is the first flight instrument which will make the helicopter descend
As always with flight instruments, never increase the collective abruptly but apply smooth movements
To establish a normal descent from straight-and-level flight at cruising airspeed:
Lower the collective pitch control
Increase right antitorque pedal pressure to maintain heading
Simultaneously apply the necessary cyclic pressure to control the airspeed.
Usually the cyclic has to be pulled
Never push down the cyclic abruptly to make the helicopter descend
Note that a helicopter can descend with a nose-up attitude.