Take a big step out to the side. As you drop down into the lunge force your knee out to the side, force your butt back, lean forward THEN when you feel like you can’t get any deeper with those movements just bend your knee a little. That will let you drop down in and get your thigh parallel to the floor. Keep both feet flat on the floor, too often I see these done with the trail leg foot rolling to the side of the foot.
A good progression depending on your population could be body weight, med ball on the chest, kettlebells, then a barbell.