how do we write a proposal to tax the transactions?
First, new governments will have to be elected; second; the nature of election finance will have to be changed to remove the stranglehold now held by wealth over politics. Then a serious program of wealth redistribution, framed as "disgorging ill gotten gains," could be devised. This would need to be coordinated on an international level.
If the choice were between surrendering 80% of your personal wealth or having to move to Burma or the Central African Republic or some other such "haven" there might be many takers. How about million dollar bounties on fugitive billionaires? Dead or alive? Might not get back the money, but it could discourage flight. Might as well dream big! :-) As the Dutch said while fighting the Spanish: "It is not necessary to have hope in order to persevere."