I'd love to be able to greet you in American Indian sign language, but the gestures were limited to only the most common words.
So many languages were spoken among the plains Indians, and most were radically different from one another, so the only way tribes could communicate among themselves was through the use of their hands. Sign language proved an effective means of communication for simple needs.
Since horses were such an important part of all tribes and a demonstration of wealth and success, it stands to reason the gesture of the index and third finger of the right hand straddling the index finger of the left was something often seen.
Trading buffalo skins for food or identifying oneself and tribe would be fine examples of times when sign language might be used. Eventually fur traders, trappers, and soldiers who came to the plains learned enough sign language to be able to freely communicate. Each tribe had their own way of introducing themselves. For example: A Crow warrior declaring his tribe would hold his fist against his forehead with the palm side out. A Commanche would imitate a snake's movement with a finger, and a member of the Sioux tribe would move their hand across their neck in a cutting motion.
Words were determined by varying factors: location, movement, handshape, and orientation.
Location = where the hand is placed...for example in front of the face as opposed to in front of the lower body.
Movement involves the way the hands move when forming the sign. Some require slashing motions while others are stationary, or move either above the head or arch to the side.
Handshape is determined just as it sounds; the shape the hand takes on when signing. For example, "I Know" reqires the hand to form the 'L' shape.
Orientation refers to the placement of the palm and the role one might play when acting as a base from which the other hand moves.
Unless you take a class and learn the ins and outs, it all sounds pretty confusing to me. So, I would recommend you learn only one sign. To describe my blog, you would demonstrate "good" by placing the right hand horizontally in front of the breast, and move it forward. That's all you need to say. :)