In the movie Bury My Heart at Wounded Knee, the character of Ohiyesa was awarded a feather in recognition of his bravery at the Battle of Little Bighorn. Although we never see exactly what act results in the earning of the feather, there are several things a young Sioux male could do in order to win such as honor.
Young men who did some heroic act in battle or in the face of the enemy would recieve such a token. Unhorsing an enemy, stealing his mount, touching him with your bare hand or killing him could all be possible ways he earned the feather. In the context of the movie, Ohiyesa could have unhorsed a soldier, stolen one of their horses as they fought dismounted or killed someone. He could have also saved the life of another warrior, as brave or honorable deeds were also rewarded in kind.
Ohiyasa would have worn the feather into battle or kept it safe until he was able to make a stick or spear to display it on. If he had been able to collect several feathers, he could have eventually created a headress, which would have marked him as a superior warrior of great bravery and skill.