I've been working on this too. Vida does ok with it but still doesnt have it completly. I lay the rein on the opposite side of the neck of the direction I want to go. If I get no response I give a little tug on the other rein. She has the right turn down but the left turn is a little slower. I keep thinking its the thick mane she has, she can't feel it so I kind of do a sawing motion with the rein.
It takes time, lots of repetition. I'm not sure if this is the proper way so if anyone else wants to add something please enlighten us both