Chazz, I used to do this when I was bored at the camera store and the technique is very simple. If I recall correctly the SX70 film is only used to do the "scroll over" method, IOW once you have the film you grab a stylus and start to paint or alter the image, it is kind of neat but this film will not separate from the substrate.
The kind you need is the film that peels apart once you have developed. I dont exactly recall the name or designation but you should not find it hard to find the information.
Once you have taken the picture and allowed the development time, you peel it apart and you put the picture in a tray with water. Some people add sodium carbonate to soften the emulsion, I never did. You wait for 4 or 5 minutes and then you start peeling apart the emulsion by one of the corners. You have to be very careful and have a delicate touch as the emulsion is very thin. Continue rubbing with your finger and work your way towards the middle from all four corners. Once the emulsion is separated comes the hard part, if you jostle the tray too hard the emulsion will stick and forms a little useless blob....so very carefully remove the backing and with your two hands grab a the emulsion by the corners and lift from the tray and place on a piece of watercolor paper or any paper of your choosing. I forgot to add all this should be done in the water or the emulsion will break.
Thats it, is simple but requires patience and care.