From "Final Exit: The Practicalities of Self-Deliverance and Assisted Suicide for the Dying" (1991) by Derek Humphry:
Hans Atrott has written to me that he knows of more than 300 people who have ended their lives with the use of cyanide to avoid further suffering. He has been present at several cases. His organization believes this is by far the best method though it has to be done with a certain sophistication.
Atrott says that correctly used, death by cyanide is by no means violent. It is quick and gentle. "Anybody who saw a correctly applied cyanide self-deliverance would want it for himself or herself," he claims. He blames cyanide's bad name on the failure in medical literature to distinguish between the properties of hydrogen cyanide (HCN) and potassium cyanide (KCN) and their differing usefulness. DGHS has learned both from medical scientists and experience that KCN alone is the useful agent in suicide. That is borne out by the stories I have researched elsewhere in the world. The recommended technique by DGHS is:
1. Take a small glass of cold tap water. (Not mineral water, nor any sort of juice or soda water because of its acidity.)
2. Stir one gram, or 1.5g at most, of KCN (potassium cyanide) into the water. (To use more than this amount would cause burning of the throat.)
3. After about five minutes the KCN is dissolved and ready to drink. It remains drinkable for several hours, but not more.
4. Once the potion is drunk, consciousness will be lost in about a minute. There will be just time to rinse out the glass (to ensure that no one else accidentally drinks from it) and lie down. But beware - a person extremely weakened by illness might black out in twenty seconds.
5. While in the coma, death will follow in fifteen minutes or at most forty-five minutes, depending on the physical strength of the person and whether the stomach is full or empty. An empty stomach promotes faster death.
6. During the coma period, the dying person will breathe heavily or snore, similar to people who have taken a lethal overdose of barbiturates.
Atrott notes that with a person who is seriously terminally ill, death is so peaceful that often doctors do not detect suicide, and sign the death certificate as being from natural causes. Of course, an autopsy would uncover this.