The way I lock mine back is this. Grab the gun as if you're going to shoot it with your right hand. With your left, grab the rear serrations. With your right hand thumb, apply pressure to the slide stop. Push to the left with your right hand while pushing the slide with your left hand to the right. When the slide is back enough the pressure from your thumb will lock the slide open.

Hold gun parallel to your chest so it's like you're trying to shoot at a left target from your chest.