I was taught to steer by just looking (no other aids) and it frightened the wits out of me. My early lessons and just looking and praying.
The RI who years later rescued me for school work told me to look (e.g. down the centre line to A when turning down the middle in trot), but also to point my tummy button in the direction I want to go. So look down the middle quite a long way in advance before the previous corner to prepare the horse and turn your tummy button at the moment of turning.
I am not sure that would work in leg yield - With due respect to eml I myself in leg yield, look straight down the school - I want the spine of the horse to stay straight and parallel to the side of the school, even though the horse is moving sideways. The time to signal with a look is for me is canter at x or on the centre line. But in the Olympics grand prix final I noticed some top riders really twisting their head from side to side in the lead changes and that did not look pretty.
I am really home sick for my dressage lessons.