Everyone has good ideas. You could try this, this is a beginner lesson. Walk your horse along the fence, lets assume the fence is to your left. Gently use your left rein to tip his nose to the fence, use your left leg to apply pressure behind the rear cinch until he is perpendicular to the fence. Do that for awhile then increase it by moving your leg to his rib cage until you get one sidepass step. Some times horse get sticky feet, you if you can keep his feet moving while you are teaching him that helps. Don't expect him to side pass fifty feet to start with.
I agree with some of you,somewhat.i don't believe in the fence,cause if you can't sidepass him in the middle,then you are not using the right amount of light adjustment with your hand and leg cues.and if you don't use or touch the same places from the ground that you do from the saddle,then why do it?is it because you have seen it on a video?
I don't use the fence technique either, BUT your horse needs to learn how to leg yield CONSISTENTLY first! When I taught a dun gelding I owned to side pass, I would leg yield him through the corners of arena. You HAVE to have some forward movement when teaching a side pass, OR your horse will jam up and back up. So, if your horse moves away from leg pressure....you are on your way to a sidepass. This is why leg yielding through corners will help...the horse already has forward movement and you are asking them to yield away from your leg laterally.
^yes I would Rob. Like I said, I've never used the fence because then THAT creates a "visual" for the horse and the horse will associate the fence and the leg cues as one...know what I mean? What happens when you take the fence away? You can be back as square one....asking for a sidepass and the horse doesn't understand.