Parsva Halasana

This pose is done through Halasana.

At the base of the pose, press the upper arms into the support. Move the chest toward the chin; lift from the armpits to the hips & lengthen the buttocks to the heels. 

Moving the right foot to the left:
Make the right upper arm dense and keep it planted into the support, roll the thighs from outer to inner; move the right foot to the left; lengthen the inner right heel away; lift the left hip up & push the left inner heel towards the right. Walk the feet further to the left whilst keeping both inner legs "glued" together and the hips level.

B.K.S Iyengar - Parsva Halasana

Finer points:
Base - moving the hands as close as possible to the support, using the hands to lift the skin to the hips and supporting the back of the rib-cage help to plant the arms firmly into the support.
Keep the the thighs firm, open the backs of the knees & knee caps lift back to the body while the inner thigh of the leg that moved passed the center-line of the body (right) extends away. Move inside of the other leg back to the body. The navel moves back to the spine. Keep the buttock skin moving towards the heels.

Utthita Parsva Hasta Padangustasana

This asana is not detailed separately from Utthita Hasta Padangustasana in "Light on Yoga" by B.K.S Iyengar & is only detailed as a variation in "Yoga - a Gem for Women" by Geeta S. Iyengar.

Easier to balance when you can hold the toe.

Method - Balancing in the Room:

Stand in Tadasana, pay acute attention to the left leg, pressing firmly the big toe joint. Place the left hand on the the hip when balancing in the middle of the room.
Bend the right leg, take hold of the right big toe, bringing the arm to the inside of the thigh, by putting the right index & middle fingers under the toe & the thumb covering the big toe nail. Take the heel close to it's own buttock.
Taken the knee away from the mid-line of the body out towards the right side ribs. Straighten the raised leg, drawing back on the kneecap, pressing the inner heel & big toe joint of the right foot away.

General points of the pose detailed.

Finer Points & Variations:

Tadasana action on the standing leg (left leg) - be firm on the inner foot, spread the toes from the big toe to the little toe, creating as much surface area at the base of the pose as possible. Descend the inner ankle to the inner heel & lifting strongly from the inner knee to the inner groin.
Re-engage this action after bring the bent leg knee up toward the body & the heel closer to it's own buttock bone. Move the flesh at the top of the thigh of the standing leg onto the front of the femur. Flex the bicep of the right arm (holding the toe of the raised leg), drawing the shoulder blade down the back and moving the base tip of the shoulder blade into the back chest. Aim to keep the side ribs on either side of the body, feeling equally long & parallel to one another. Lift the sternum to the face; relax the tongue. With this action bring the buttock bones more into horizontal alignment and allowing the buttock flesh of the raised leg to descend from the head of the buttocks to the buttock bone. The tailbone remains broad and pointing toward the floor, moving into the body.
Open the inner groin of the raised leg, moving the knee out to the side, lengthen inner groin to the inner knee; then straighten the leg, maintaining the descending action from the head of the now extended leg buttock to the buttock bone, and continue that extension to the mid-heel, lengthen from that mid-point of the heel, "drawing a line" bisecting the sole of the raised foot & allowing the ball of the foot - the toes - to spread away from one another & away from the heel. Draw back on the kneecap of the raised leg.
Firm the top & bottom of the elbow of the arm (right) extended to the raised foot. Keep the shoulder blade on that raised leg side moving into the back body, supporting the chest.

Using Supports:

Balance is always an issue for the less practiced body & as always simple is best to mitigate unsteadiness. Stand three to four inches from the wall in Tadasana and bring the hand to the standing leg hip, & press the elbow into the wall behind you. This gives real people with buttocks space for them to "operate". Both buttocks change shape when weight is put onto or the other foot, they perform slightly different tasks.

A belt can be used to put around the foot if the big toe is inaccessible, due to tight hamstrings. The correct action on the shoulder blades help with this. Aim to keep both legs straight & the backs of the knees open, before attaining height in the raised leg foot.

Room for improvement - always!

Namaskar.