Shooting tips
While I am certainly no expert shooter, if you are new to this style of shooting there are a few tips I can offer from my own personal experience to get you off to a good start.
With this style of shooting the projectile is thrown over the top of the forks rather than through the forks. As such when learning there is a high chance of the projectile striking the fork and possibly damaging the frame. Until you are used to this style I would recommend taking your first shots with a relatively inert ammo such as the plastic ammo by Barnett or paint balls. I would definately avoid marbles to start with as if they hit the fork they can explode in a shower of glass! When you become more profficient I would recommend at least 9mm steel balls and for hunting a heavier projectile such as 11-15mm steel or 12mm lead.
Grip style
With this style of catapult the frame is gripped with the fore finger wrapped around one fork while the thumb is braced against the other. The bottom three fingers are held around the main handle with the little finger held quite loose allowing the handle to move forward slightly at the bottom as the bands are pulled.
With the ambidextrous hunter some users find it comfortable for the thumb to also wrap around the fork instead of just braced against the frame.
Avoiding fork strike
Try to keep the frame perpendicular to the direction of pull and avoid tilting the frame forwards or backwards. As the pouch is released let your wrist go loose and allow the momentum of the bands and projectile to flick the wrist forward.It may help to practise pulling the bands while in front of a mirror or getting a friend to check your form.
Grips to avoid
Aiming
I tend to hold the catapult horizontally and try to get the bands lined up vertically with my eye, a bit like iron sights on a rifle, with the bands beneath my target. I find this way I only then need to adjust my shots for elevation, with my exact aim point usually ending up about one to two inches above the fork tip. The exact aim point will vary from person to person and which eye you use to aim with. Make sure the projectile is positioned in the centre of the pouch and try to draw to the same point each time (I tend to use the corner of my mouth).
Avoiding hand slap
With this style of catapult as your hand is positioned so close to the bands there is a chance that when the bands contract after the shot the pouch can hit your hand, which after a few shots can become quite painful. The best way to avoid this is to let your wrist flick forward as described above, but also using heavier ammo works well. In the early stages you may wish to wear a glove until you pick up the technique.