+ I'll hazard an opinion --

It may be a good rule of thumb to say that a person has excommunicated him or herself by ceasing to take part in the Divine Mysteries -- by ceasing to commune for some period of time.  But that's pretty fuzzy, and can't really be applied in a legalistic way. As I understand it, some Orthodox  Churches developed a piety so rigorous that for  most people frequent communion simply became too difficult. Also, some felt an exaggerated sense of being unworthy to commune. But these things in no way involved a rejection of Orthodoxy, and it's now increasingly recognized  that frequent communion is the norm.

Heresy is another matter. Heresy is by definition counter-Orthodox. So if a person deliberately rejects Orthodox teaching and practice and joins himself to some heretical sect he of course ceases to be Orthodox, and cannot be "counted" -- not that anyone's counting.

I don't think there is or needs to be any actual procedure for leaving the Church. I suppose that if someone wants to be really formal they might write a sort of letter of self-excommunication. And I would suppose that a Bishop who receives such a letter would simply have to respect that person's choice. You can't accept people as Orthodox if they themselves choose to reject the Orthodox faith.

H