Apple seeds average around 0.6mg hydrogen cyanide (HCN) per gram of dry seed. Since the lethal dose of HCN is estimated to be around 50mg, you will need around 85 grams of dry seeds. This is around half a cup... it requires a lot of apples.
1. Plants are variable; eat enough - at least 3 times the minimum dose; Cyanide is not a drug on which to skimp, since it can cause brain damage in sublethal doses.
2. The HCN must be liberated from the sugar it's chemically attached to. This occurs when the moistened seed is crushed, releasing an enzyme, emulsin, which does the job. You need to crush and eat the seeds fairly quickly, both to avoid evapouration of cyanide from the crushed seeds and so as not to lose consciousness before ingesting a lethal dose. A blender or a coffee grinder would be a good way to break up the seeds.
3. Effects are fastest if the stomach is empty and gastric acidity high. With minimally lethal doses, death may take up to an hour.
4. Some claim death by cyanide is painless and quick, others that it is painful and quick. Cyanide is commonly used by suicidal chemists but rarely by suicidal physicians.