No, not completely painless. Cyanosis, nausea and perhaps headaches are to be expected. Hopefully the latter two can be avoided by using anti-emetics and acid reducers that will both combat the nausea directly as well as speed up the process so that you're unconsciousness by the time the worst symptoms set in. You'll also have to lie there for a while knowing that you're about to die, which may be unpleasant enough on its own to some.
The hypoxia works as I explained above: the level of blood that can carry oxygen is reduced as the nitrite is absorbed into your blood. When the methemoglobin in your blood reaches a certain level, the amount of oxygen delivered to your cells will be insufficient.