I second Kelly on that. Her programs are no longer available to buy, but you can find some on youtube.
To properly teach your horse a balanced sidepass, you need to already have control of their hindquarters and forequarters and a solid back. There are two ways to approach it depending on where your horse is most comfortable. Most horses are easiest to start with the hindquarters. You would step toward their hindquarters just as if it were a turn on the forehand, then once you have them moving lightly, step toward their forequarters and ask for a pivot step. Then as soon as you get that, back up in the opposite direction and reward them. That keeps them from pushing forward.
The other way would be to start with a pivot and then ask for them to yield hindquarters.
You'll continue that, trying to ask for a little more each time. At first it will be a seesaw motion, hindquarter, forequarter, on both sides, always backing the opposite way after each time, but after a day or two it will start to flow. As you progress, you'll be able to refine your cues until they are almost unnoticeable and you can stand in front.
Do try to incorporate a subtle vocal cue that is different from anything else so they don't confuse their pivot with sidepassing.
I hope this helps and makes sense!
All of the above. Plus it really helps the horse if you place him in front of a fence while initially working on a side pass. It helps to block any forward motion and turn that forward motion into "sideways" because there is nowhere else to go and sets them up to succeed. I use the word "over" as a cue. When I first start teaching them, if I get one or two really good "cross over" steps I give a reward and quit while I'm ahead. I'd rather get a couple of good quality steps than a bunch of sloppy ones.