This is a theoretically simple method of suicide, where the cause of death is hypoxia (lack of oxygen to the brain and body).
Given many drugs do not, on their own, induce death (especially if the dose is below the minimum lethal dose), the idea is to ingest drugs that will cause unconsciousness, and then for the plastic bag to cause death by hypoxia, as, with no fresh oxygen in the bag, carbon dioxide is inhaled eventually causing death.
Care must be taken to use a bag that will not tear if the body is convulsing, or during semi consciousness.
Nitschke & Stewart1 and Derek Humphry2 recommend the sort of oven bag you roast a turkey in. Thin plastic bags where the plastic will easily stick to the face are not recommended, although Stone3 mentions that wearing a hat with a brim can avoid this problem. The bigger the bag, the slower the asphyxiation. Whatever the bag size though, there does need to be some seal around the neck. It does not need to be tight - an elastic band or a loop of elastic should do the trick.
The major drawback of this method is running out of oxygen in the bag whilst still conscious. Humans, whatever their mental state, have an underlying strong desire to stay alive, so are likely to tear or remove the bag in this situation. A bigger bag may be the answer, although it is always hard to know how long it will take for any given drug dose to render unconsciousness, and whether the oxygen in the proposed bag size will last more or less than that time. In Final Exit Derek Humphry talks about elasticating the bag, then holding the bottom open so breathing is easy until consciousness is lost, when the elastic would gently close the bag around the head.
Stone3 states that a 30 gallon trash bag should have around 30 minutes of air in it, although in his own tests he stated that after only 15 minutes carbon dioxide build up was sufficiently high for his breathing rate to more than triple and be uncomfortable enough to want to remove the bag. On Alt Suicide Holiday4 it is stated that even when people start to become unconscious, they can remove the bag from their head, thus making this method prone to failure.
Another option is to try and place the bag over the head as the drugs are starting to take effect, although this is prone to failing to get the bag over the head in time, or still running out of oxygen. A small tent that has all vents sealed may offer a viable alternative to a bag, and have a fair amount of oxygen to last for a number of hours until the drugs take effect, although in this case it really would be important to ensure that no air can leak into the tent once inside it.
It is probably advisable to do testing with different types of bag/tent before any suicide attempt to get a gauge for how long any given bag/tent can be breathed in before it becomes uncomfortable. Then ensure that whatever drugs are taken will cause unconsciousness in less than that time.
This method does take time for death to occur. The time all depends on how much air is in the bag/tent, and potentially what drugs are being used. Whilst discovery and interruption of the attempt within a relatively short time frame may not cause any permanent damage, discovery later in to the process may result in permanent brain damage. For this reason, it is absolutely essential that there is no chance of being discovered for a period of at least a few of hours.
So whilst this method is potentially lethal, generally painless, and will leave a peaceful looking body, it does have a number of drawbacks to carry it out effectively. Important considerations are a bag with enough oxygen to last until the drugs cause unconsciousness, ideally a bag that is comfortable to breath in whist conscious, drugs that will reliably cause unconsciousness, and to ensure there is no chance of being discovered.
Although this method is mentioned as potentially lethal in Final Exit, and mentioned in many forums, it is not mentioned in Peaceful Pill Handbook, and Alt Suicide Holiday states that posts on their newsgroup consistently report this method as failing, and advise against using it. There are certainly more reliable methods.
Anyone seriously considering this method of suicide is advised to read Help me first.
Dr Phillip Nitschke with Dr Fiona Stewart, The Peaceful Pill eHandbook, revised 10 October 2009.
D Humphry, Final Exit: The Practicalities of Self-Deliverance and Assisted Suicide for the Dying, 2002.
Geo Stone, Suicide and Attempted Suicide, 1999.
Alt Suicide Holiday website, http://ash2.wikkii.com/wiki/Exit_(plastic)_bag_with_sedatives.