Military historians regard the invention of the stirrup as a major development in the art of warfare. If you were involved in mounted warfare and were trying to whack some guy on the head, it really helped to have the stability and security that stirrups provide. Fortunately society has stopped using horses for warfare; equally fortunately, the stirrup has stayed with us. It is fortunate because, for a rider, the stirrups are the ground.
Because I want you to be well "grounded" in your technique, I always make this point about the role of your stirrups. But stirrups or no stirrups, if you don't have a good lower-leg position, you literally don't have a leg to stand on. I have been teaching people to ride over fences for quite a while now, and I am firmly convinced that you cannot overstate the importance of lower-leg position.
Anything of importance is built from the foundation up--including the lower-leg position. We are talking about the jumping lower-leg position, so we have to adjust our stirrup leathers correctly for jumping. I want my riders to have about a 90-degree angle behind their knees when seated. The best way to achieve this from the saddle is to take both feet out of the stirrups and then adjust your stirrup leathers so that the "tread" of the stirrup (the part that you tread upon) touches you at, or just above, your anklebone.
Build a Solid Foundation
Because your stirrups are your ground, let's start there: Once you have adjusted your stirrup leathers for jumping, put your foot in the stirrup. Don't jam it into the stirrup; place it there very carefully and purposefully. Put the ball of your foot on the tread of the stirrup. While your horse is at the halt, stand up and sink your weight into your ankles. If you have adjusted your stirrups correctly, you should be able to rise in your stirrups without effort. Practice this a few times, as this transition from a light three-point to a two-point is the basic movement you should make when jumping . but I am getting ahead of myself. We are still building our foundation.
You are now seated in the saddle at the halt with the ball of your foot on the tread of the stirrup. Allow the tread to assume a slight diagonal angle to your foot, with your little toe against the outside "branch" of the stirrup. The branch of the stirrup is the long metal arm that connects the tread to the stirrup eye. If it is made of nickel or stainless steel, it is, of course, always clean and highly polished. As far as I am concerned, metal polish and saddle soap never go out of style. Remember that there is no such thing as an unimportant detail for a serious rider. (See how hard it is for me to help you build a solid foundation? I keep getting distracted, because I find absolutely anything about horses fascinating.)
While I am distracted, I might make the point that some jumping experts place their big toe against the stirrup's inside branch, rather than their little toe against the outside branch. In theory, little toe against the outside branch is French, while big toe against the inside branch is Italian, but that comes under the heading of "too much information."
My own preference is for the little toe to rest against the outside branch, but that is a preference rather than a requirement. My observation is that a rider's foot will naturally adapt to the side of the stirrup that is most comfortable for that rider's conformation. The only really important consideration is that you must be both symmetrical and consistent. I have been told by proponents of both techniques that I must do only one or the other, but I have found that both techniques work. I think the important part is that you find out which technique suits you best, and then use that technique.
Vertical vs. Perpendicular
Once you have taken care of all these details, rise and put the weight in your ankles again. This time, pay attention to the angle that your foot forms with your horse's body. This angle should be the same angle as that with which you walk. I have heard trainers say that a rider's foot must be parallel to her horse's body or it must be at a 30-degree angle. I disagree. There are some absolutes in riding, and we are going to discuss one in a moment, but this is not one of them. The angle of your foot to your horse must be natural, so that you are relaxed and comfortable using your ankles as shock-absorbing mechanisms.