In May 1942, the first 29 Navajo recruits were sent to boot camp for training. There they made a dictionary and words for military terms. They all sat in a classroom and made suggestions. They would tell the trainer and he would quiz them on the words ("Navajo Code Talkers"). They had to be sure they knew them well enough. These words had to be memorized during training. After training, the code-talkers went to a Marine unit, where they would talk, make orders, or give battle arrangements over the phone or the radio. Below is a picture of the very first Navajos that fought.
http://library.thinkquest.org/J002073F/images/platoon.jpg