Sidepass is the last movement you should teach. First, teach turn on the forehand and turn on the haunches. On the ground first is best. A sidepass is basically a turn on the forehand and a turn on the haunches at the same time, so it's much easier for your horse to comprehend if you go to kindergarden before middle school.
I don't do the fence thing; it's never worked for me
. It just made my horse want to back up and get frustrated. I started by walking in a smallish circle. Move you inside leg very slightly behind the girth (but not as far back as you use to cue for a turn on the forehand). Nudge nudge. Your horse should move away and make the circle bigger. Eventually have your horse move sideways while traveling in a straight line.
Gradually ask for more side movement than forward movement. "Close the front door" by pulling back slightly on your reins. Use your left leg to nudge slightly behind the girth and pull your right rein up and across the withers. Hold you left rein steady, with a little contact to prevent your horse from overbending to the right.
You should do a sidepass-y thing. It probably won't be perfectly sideways, but it'll get there. With practice, it'll be almost perfectly to the side.