The first thing I would do is take a few lessons with a trainer on a horse that already does what you want to learn to do. If you've never ridden a correct sidepass or leg yield or turn on the forehand it will be much harder to teach...especially without a qualified instructor on the ground.
To teach a horse to sidepass you can do a few things. I would start at a walk, ride off of the trail or the fence 3-4 feet and ask the horse to leg yield over to the wall by squeezing with your inside leg as the inside hind foot is acoming off the ground...that's the foot you want to influence and if you ask the horse to step over while that foot is on the ground or almost on the ground you won't get much in the way of results.
Focus on quality of steps, not quantity. The horse needs to be moving forward and sideways...a lot of forward, a little sideways. If you get one step of sideways and then three of just forward that's fine. Praise your horse when he steps over, say good boy.
After he starts getting it and moving over more readily 3-4 feet from the rail move 5-6 feet away, keep working on your timing and the horse only very slightly bent to the inside. As he gets better you can up the pressure, ask for more sideways than previously. Once the straight lines are going well you can do spiral circles, start on a small circle and ask your horse to move sideways while still staying on a circle, timing your aids correctly each time, once you've moved out to a large circle you can change your aids and go the other way too.
After you guys are doing well while moving at the walk and the trot I would begin working on it from a stop. Ask the horse to step sideways, using the same aids as you did for moving leg yields, if the horse just shifts its weight in the correct direction (not even pick up a foot) then praise like crazy and relax. Ask again. Praise like crazy and relax when he shifts. If he's relaxed, ask for a foot moving to the side. Praise like crazy and walk forward. If he's antsy just standing and shifting I would walk forward a bit after each ask.
As he starts responding you can ask for more sideways before relieving the pressure and praising. I would alternate standing for a while after relaxing and walking forward so that he doesn't anticipate walking right away and keeps thinking forward (not backwards).