Uraho,
You will need two passport sized photos with a white background which you can get done easily at any of the photo express shops along the road and they will cost about 1000-2000 RWF. All payments for Visa's are in RWF, which depends on the type of Visa needed. You need to go around the back of the office building to the payment office and pay for the kind of Visa you need FIRST and then go to the Office to hand in your receipts with all your other forms. All forms can be downloaded off the immigration web page: www.migration.gov.rw, you can even now apply for Visa on line but I had no luck with that route. You may need a letter from the agency you are there working with, a copy of a local contacts passport and address, and you have to write a letter of request to the Gov. General explaining why you want to stay.
This is what I needed, I am Canada but most of this was pretty standard for the type of Visa a required which was simply an extension on a tourist visa. The office is in Kacyiru Sector, near Hotel Umubano - just hope on a moto and tell them immigration or Umubano and you will be on your way! The work week in Rwanda ends friday at noon so keep that in mind when you go to drop off and pick up your papers. When you walk into the Visa office you have to go to the small white machine on the counter, on the left hand side of the office clerks, and get your number!! Then take a seat and wait to be called, which is surprisingly quick!!
You have a window of time for applying after your entry Visa has expired which is 5 days or so and after that you have to pay a fee for late application.
Good Luck and hope that was helpful! I know I could have used some help while I was figuring it all out!!
Amanda