Give as much information as possible but be clear and concise. You need to explain and give evidence about when and how the circumstances impacted on your academic performance.
It is a good idea to keep your school regularly informed of any personal matters affecting your studies. You are in charge of your personal information and what you do with it, so boards of examiners will only take into account what you choose to disclose. The only way that exam boards can be notified about student's circumstances is via the extenuating circumstances system. What this means is that you must fill in a form, even though you might have discussed problems with your tutor or another member of staff.
Just speaking to your personal tutor is not enough, you must submit a form!
It is possible to either email forms and evidence to your school office or hand in paper copies. Some schools also have online submission systems. If you're confused about what to do, ask at your school office.
Please note that to self-certify your absence from an exam you will also need to contact your school office on the day of the exam before the exam starts to let them know you won't be there.