Talk to your close relatives and friends, and / or health and social care professionals about the sort of medical treatment and care which you would or would not want to receive, and under which circumstances. If you don't have an advance decision and you are unable to decide about what is being offered, then doctors and nurses will ask your immediate family or friends about your wishes, so it's a good idea to make sure they know what you would want.
In most situations an advance decision does not need to be in writing, but putting it in writing should ensure that medical staff will be made aware of it if required.
But if an advance decision is about life-sustaining treatment it has to be in writing and include a statement that the decision applies even when your life is at risk, and be signed and witnessed.
If you make an advance decision, you should let your immediate family know about it, and also let your doctor know so that it will appear on your medical records if you need urgent treatment.