To Rank UP, a member must complete a certain number of missions. These missions amounts get higher as you began to journey and evolve ranks.
Genin to Chuunin: Chuunin Exams, or 5 D-Rank Missions and 3 C-Rank Missions
Chuunin to Special Jounin: 6 D-Rank Missions, 4 C-Rank Missions, and 3 B-Rank Missions
Special Jounin to Jounin: 7 D-Rank Missions, 5 C-Rank Missions, 4 B-Rank Missions, 2 A-Rank Missions, and Kage Approval
Jounin to Sannin: 8 D-Rank Missions, 6 C-Rank Missions, 5 B-Rank, 3 A-Rank Missions, 1 S-Rank Mission, and Kage approval
As you rank up, the number of missions you do, do follow you. Which means say you are ranking up from Chuunin to Special Jounin. You would only need to do 1 more D-Rank, 1 more C-Rank and 3 B-Rank Missions.
Here are a few alternative ways to rank up in this Naruto World:
Academy Student: Join the ninja world
Genin: Pass the Academy Exam (Written)
Chunnin: Pass the Chunnin Exam (Written and Arena)
Special Jounin: Pass Special Jounin Exam (Written and Spar)
Jounin: Pass Jounin Exam (Written, Spar, and Arena)
Anbu: Chosen directly by Kage (Be a Jounin or Higher)
Sannin: Chosen directly by Kage (Be Special Jounin or Higher)
All exams are given by a high ranking ninja or NPCed by a staff member.
For missing ninja:
D-Ranked: Kill a Genin or Kidnap a Genin/ 2 Academy
C-Ranked: Kill a Chunnin or Kidnap a Chunnin/ 2 Genin
B-Ranked: Kill a Jounin or Kidnap a Jounin
A-Ranked: Kill a Anbu/ Sannin or Kidnap 3 Anbu
S-Ranked: Kill a Kage/ 2 Sannin/ 4 Anbu or Destroy a village of any caliper