Simple.
1) You just place an uncut leader under the spring clip of the rotating arm .
2) Switch on motor.
3) Press the button.
The arm rotates over the top sprocket, the lens assembly hinges down. The film is here now. Under the sound rollers. The rear sprocket retainer opens as the film passes through. Now under the rear roller. The arm stops at 12 o'clock.
Let enough film pass for attachment to rear spool and switch off motor.
Simple.
You can find one operating on YouTube.