Remove the MAIN BLADES from the helicopter
Secure the helicopter so it cannot move.
For a nonmicro heli, put a wooden beam or two-by-four through the skids and place a brick or other heavy object on each end of the beam. For a micro heli, put a piece of wood through the skids and clamp the wood to a table on both sides. Also, make sure the tail rotor is away from any objects.
If you have a Futaba transmitter and a Castle Creations or Schulze ESC, then you will need to reverse the throttle for this step. The throttle reverse may be necessary for other transmitter and controller combinations as well.
Install a battery in your heli, then follow the ESC directions to arm the ESC. If your ESC does not arm, then you may need to reverse or unreverse the throttle direction.
Give just enough throttle on your transmitter to make the head spin a little.
If your main rotor blades are spinning backwards, and
If you have a brushed motor and a brushed motor controller, then you need to swap the two motor wires.
If you have a diode wired to your motor, then be sure to swap this also because the grey band needs to be on the positive (red) wire to the motor.
If you have a sensored or sensorless brushless motor and a sensorless brushless motor controller, then you should swap any two of the three motor wires.
If you have a sensored brushless motor and a sensored brushless motor controller, then you should consult your motor manufacturer. This usually requires the motor to be sent back to the factory to have the motor retimed.