Requirements -

Ninja Rank of at least C-rank. Completed

Five tiers of stats trained above Ninja Rank. B-3 in both Perception and dexterity and B-2 Speed

Four completed missions of B-rank or higher.
1) http://www.narutosaga.com/t30461-the-otter-kills-the-wolf-b-ranked-mission-private
2) http://www.narutosaga.com/t30443-combat-medic-dispatch-a-rocky-start-to-a-medical-career
3) http://www.narutosaga.com/t30461-the-otter-kills-the-wolf-b-ranked-mission-private
4) http://www.narutosaga.com/t30519-infiltration-and-information-a-new-chuunin-emerges

Can be granted as part of a Chūnin Exams. (nope)