I would start by teaching the horse to turn on the forehand so that they can get used to the idea of you controlling the sideways movement of their body. Just stand at the horse's side, facing the shoulder and take the hand closest to the hindquarters, tap them on the hip, and say 'swing.' If the horse gives you even one step, stop and walk her around for a minute and then go back to it. In a few short days, you will be able to get 180-360 turns on the fore.
Once that is done, get the horse turning on the haunch. Stand in the same position as above, but this time tap the point of the shoulder and make a 'kissing' sound. Again, one step should earn a reward. This takes a little longer to teach than the turn on the haunch. So, be patient.
Once you can turn on the fore and turn on the haunch, the horse is used to you controlling the movenets of both the front and hind ends. Now, stand in the same position that you've been working in, but the pressure this time goes at the girth area. Keep the head and neck in line with each other and ask the horse to step sideways while saying 'side'. This usually will happen, in the beginning, by asking for and getting one step of a turn on the fore (the hindquarters step to the side) and one step of a turn on the haunch (the forehand step to the side). It's not super smooth at first, but the horse will soon realize what you are asking and start moving both ends at the same time with your cue of 'side'.
Hope that helps.