Turn your left foot out at a 60 - to 90 - degree angle, adjusting it so that your left inner arch is in line with your right heel. (yogajournal.com)
But, basically, we have 3 arches of the foot that create this dome - like infrastructure: a large inner arch, a smaller arch on the outside of the foot, and the transverse arch which goes from the ball of the foot over to the pinky side. (mindbodygreen.com)
Externally rotate your left leg so that your left foot is parallel to the long side of the mat, with your left heel in line with your right inner arch. (yogajournal.com)