Many of my students often find this pose quite challenging when it was first introduced. This is the alignment cues that I use for my teaching in this video.
Below is an anatomic explanation of this pose.
Utthita Trikonasana Triangle Pose – The intention is to initiate the movement primarily from the pelvis while maintaining the spine in neutral extension.
In order to tilt the pelvis sideways over the leg, we need to actively lengthen the back leg, including the muscles that across the side of the pelvis, the outer hip and the outer knee. To do so, we press into the outer heel of the back foot, and press the front thigh (femur) back.
As most of the weight of the torso falls over the front leg, we have to be careful that the knee is not hyper extended, the front kneecap is lifted and pointed forward. In order to do so, we externally rotate the thigh of the front leg into its hip socket, so that the inner groin, inner knee and inner ankle are aligned with each other.