It would be a lot easier if the global population stabilized at around 1 billion. It's conceivable we could get down to that by bringing 3rd world areas up to 1st world standards in terms of women's rights, access to birth control, education, standard of living, etc., since developed nations have had declining birth rates for quite a while. But it's not a cheap or popular idea & would take several generations anyway.

They don’t need to sign up for it. It will happen regardless.