Most advance decisions can be made verbally to your doctor. However, if you are making an advance decision to refuse lifesaving treatment then this must be in writing, signed and dated. It is often better to put any advance decision in writing just in case.
If a doctor is aware of an advance decision and it is valid and applicable then they are legally bound to follow it regardless of whether they think it is in your best interests. To make an advance decision you must be over the age of 18 and have mental capacity at the time of making the advance decision.