The first important point about position on a side-saddle is that the rider should look elegant. (It is not always simple!)
Roger Philpot, one of the best teacher says:
1. The rider should look exactly the same as the astride rider when viewed form behind. In other words, your weight must still be positioned centrally over the horse's back and your spine in line with his.
2. Upper body and head: Both should be held straight and upright, as in astride riding, with shoulders and hips level and square to those of the horse.
3. Seat: You should rest slightly more weight on your right seat bone, so that the outside of your thigh is in contact with the saddle. Do not fall or leaning to the right.
4. Right leg: This hangs almost perpendicular from the knee, the toe pointing slightly down and in towards the horse's shoulder. Brace the outside calf muscles against the saddle. This helps to tense your knee, which, in turn, pulls you deeper into the saddle. You should be able to place one or two fingers between the back of the right knee and the hunting head.
5. Left leg: The stirrup leather is the right length when you can insert the palm of your hand between the leaping head and your left thigh, with your foot in the iron and the leather hanging perpendicular to the ground. You will find that the same length is about the same as for riding astride. Keep your inner thigh and knee in close contact with the saddle. As with astride riding. let the weight fall down into the heel and turn the toe slightly out to the left, rather than trying to keep it parallel to the horse. This helps to keep the knee close to the saddle.
6. Hands: Hold the reins in the usual way. The height of your hands will depend on the head carriage of your horse. Perhaps, you will find you have to hold your hands a little higher than in astride riding, because of the construction of the saddle. Attention, a straight line should still run from your elbows, through the hands and reins to the bit.