The Spiderman Stretch can be used to "open up the hip" into better external rotation and flexion.
We are doing this in a lunging pattern because it's easier to isolate one side at a time.
Skip to the 1:27 mark here for a video demonstration.
In my years working with people's back and hip conditions, it's extremely common to find single-sided lack of movement in this position.
Let's work for symmetry.
Drive the front hip externally with the elbow.
Keep a long spine; rounding will only add to poor posture when squatting.
The intention is for the front hip, so don't over stretch the back one.