This is another pose that looks easy but there is a lot going on and is great for starting to open those hips.
Take your legs to a wide stance
The foot at the front of the mat should be pointing to the top end of the mat (toes first)
The back leg the foot wants to be a a 45 degree angle
Start off with both legs straight and the heel of the front foot should line up with the instep of the back foot
The shoulders and hips need to be facing the side of the mat and stay in that direction
Take the arms out wide and hold at shoulder height palms facing down.  These arms should be firm and solid not floppy and all over the place.
Take your gaze down the middle finger of the front hand
Take a deep breath in and on the out breath bend the front leg keep the knee going in the direction of the toes try to avoid the leg caving inwards.
Keeping the line of the front leg will open the hip and do not let the back leg roll round try to roll in outwards thus opening that hip
Bend the front leg as low as you can but if the knee goes over the ankle widen your stance.
Keep a check on that back arm it tends to wander.