The College Board provides testing accommodations for students with a documented need. They are obtained by request, not by asking on the day.
Which accommodations
Extra time, additional or extended breaks, testing in a separate room, large print, a screen reader and similar arrangements.
Which accommodation is appropriate is determined by the documentation; the student does not choose.
How to request
The request is usually made through the school's accommodations coordinator, who enters the student's file and documents into the system.
For a student testing outside a school the process differs, and the details are read from the College Board's own pages.
Documentation
A current assessment and diagnostic report is required. How old the documentation may be and what it must contain are governed by rules.
Incomplete documentation is the most common reason for refusal, and correcting it starts a new waiting period.
Timing is critical
Evaluation takes weeks. A request made close to the test date will not reach that test.
⛔ An accommodation cannot be used before it is approved. A candidate who uses unapproved extra time has their result invalidated.
So the request is started months before the intended test.
Once approved
Approval generally carries to later College Board exams, meaning it can be used for AP exams too. That is the second benefit of applying early.
During registration
Approved accommodations are not applied automatically at registration; the accommodation identifier has to be entered. When that step is skipped, the student is placed in a standard room on test day.