Flatwork Basics - How to turn correctly in walk
You need active hind legs, controlling the outside shoulder and riding leg to hand.
Look ahead at the line you wish to ride.
It is helpful to have poles or cones on the ground to create a channel to work with and practice.
You need to be sitting tall, weight equal in your stirrups and do not lean in as you make the turn as this will unbalance the horse.
In this video the first turn was late and led to an over correction which unbalanced the horse.
The second turn was abrupt and tight which made us unbalanced on the straight line.
The third turn is smooth, correct and straight.