The spear will automatically become available early in the game. Once you get it, go to an enemy outpost ("Kapal Outpost" to the east of the village is recommended). Use the owl to place a waypoint on an idle enemy to view the exact distance. When the waypoint is 50+ feet away, aim a little bit over the enemy's head and throw the spear. There are also enemies randomly roaming through the woods -- so there are many possible locations to do this at. The early outposts work best because the enemies do not have any armor and will die in one hit from the spear. Note: If you fail, simply let the enemies kill you and you will respawn in front of the camp and can try again.