how do I open door panel to replace speakers

You will have to remove any panel screws (arm-rest, door handles, etc) and then pry the door panel from the frame. You do this by using a flat head screwdriver to pry the retainer clips out of the door frame while not damaging the door panel (usually cardboard). Installation is done in reverse.

I recommend using proper gm upholstery tool, instead of gouging up paint work and getting rust problem later. But if ya wanna use a screw driver, at least your rusty door will sound good - ha ha