If you are staying more than 90 days, you must get an official letter from your host institution. This letter must explain your reason for staying in Mongolia and must request long-term residency permission after you have acquired your visa. Two copies are needed. One to the Ministry of Education and one copy to the Immigration office. A contract (if you have any) with your host institution may be required at the MoE (so it a good idea to have one made). In addition, you will need to do the following:
Acquire a letter of proof of your local address from your local municipality (Oron suutsni khoroo)
Go to the Immigration office and purchase a form to fill out.
Prepare one copy of a recent identification photo (approx.1.5 x 2.5 inch).
Make a copy of your passport identification page (at the Immigration office, they do copies for a nominal fee).
Finally, go back to the Immigration office with the following: Passport, identification photo, completed registration form, letter with proof of residency, letter of visa sponsorship from MoE, and + 31,500₮ (for the permit).