When should I take this step?
After you get an offer of admission from the school that you want to accept.

What happen if I forget to write one of these letters?
If you forget to send a letter to the school you want to attend, they may leave your name off of lists of students they are expecting to attend international student orientation or other events. In the worst case, they may assume you have not accepted their offer, and then you might have trouble getting registered for classes.

If you forget to inform a school that you will not be attending, they may continue sending letters to your home address asking you when they can expect you. It is best just to make certain that you send all of the necessary letters.

Why do I need to tell the schools whether or not I am accepting their offer?
Courtesy. Schools admit more students than they expect to have accept their offers of admission. They plan schedules and budgets partly on the number of students they expect to have attending each year. By informing schools that you won't be attending, you help them plan more accurately how to spend money and where to locate classes.

What does a school do when I accept their offer?
One of the most important things they will do is send you an I-20 or an IAP-64, depending on your visa classification (F-1 and M-1 visa holder use the I-20, and J-1 visa holders use the IAP-64). See our visa page (step 12) for more information about visa classifications. Your I-20 or IAP-64 will be one of the most important parts of your visa application.

The other thing that schools do when they get your letter accepting their offer is prepare for your arrival. There are a lot of materials they will prepare before you arrive, and they can do it much easier if they know you are coming well in advance.