Brad's Iyengar Yoga Notebook
For the Gomukhasana arm position, the classical way to enter the pose is by raising your top arm first, and then you can use your opposite arm to roll your upper arm inward.   Turn your upper arm triceps inward as in Urdhva Hastasana.
Next take your lower arm to your side and then up your back as far as possible with the back of your hand touching the back of your torso.   At this point, you may reach around your back and pull your lower elbow inward with your upper hand to help raise your rear hand further up your back.   Then return your upper arm to its position straight upward.
Next bend your upper arm down to grasp the hand of your lower arm behind your back.   It is perfectly reasonable to use a strap between your hands if your hands do not come together.
After you have grasped your hands (or the strap), that is when most of your work begins.   Lengthen both sides of your torso upward evenly as in Tadasana.   The upper arm side of the torso tends to lengthen more than the lower arm side which tends to scrunch up -- see that it does not happen.   Do not allow your belly to push forward.   The upper arm side of your torso should feel a lengthening action, whereas the lower arm side of your torso should broaden to the side.
Your upper arm from your shoulder to your elbow should be vertical, pointing straight up toward the ceiling, not pulled behind your head like a pillow.   The upper elbow will tend to sag forward.   Stretch it actively up toward the ceiling.   Your head should be perfectly upright just as in Tadasana, not convoluted in some position to try to get out of the way of your arms.
Don't just grab your fingers and hang out in the pose.   Actively stretch your arms apart, your lower arm pulling downward and your upper arm pulling upward, and use that traction, that opposing action, to open your chest.   Draw your shoulder blades forward into your body to facilitate opening your chest.