Horse should maintain the regular walk 4 beat rhythm (unless of course you're going straight in at canter
)
Some people ask from a shoulder in some from a travers. Basically halt halt to stop the forward movement and apply outside leg to ask the shoulders to step 'round'.
Personally I find it easier to teach the horse travers on circles first and decrease the size.
Horse must stay in front of the leg at all times and if you start to loose the forwardness then walk out of it and restart. At no time should the horse step back.
Hope that helps, I'm useless at explaining when I'm not on the horse
I prefer to ride it from haunches in. Collect the walk with your body, look back over your inside shoulder whilst asking the horse to move around with your outside leg. Make sure you dont lose the rhythm of the walk.
Thanks I was told to do it from a shoulder in but I actually find it easier from haunches in. Should it feel sort of like a very squashed up half pass??
I also ride it from travers... really collect the walk then open my inside rein and then encourage her around with my outside leg. You must keep the energy though as you need to keep the four time rhythm of the walk. Sometime, if it ends up a bit big, I tend to half pass back to the track for just a few strides.
I have my outside leg behind the girth to keep the hindlegs active and stepping through, and inside leg on the girth to keep the bend and stop the horse falling in.