Pop out the rear pin, and using some force, you can pop the receiver open.
Best way is to place the gun in your lap, right side facing you, handle towards your stomach, barrel aiming to your right.
Wrap your left fingers around the buffer tube, and use your thumb to push up on the forward-assist knob. Might need to use your right hand to help push up on the forward-assist knob. Use your left hand's fingers to maintain downward pressure on the buffer tube.
It's a safer way than "slapping" it open like some videos show. Yeah, you can do it to a real steel gun, but it doesn't mean you *SHOULD* do it. Be gentle with anything and it will last longer.