The best possible way to get this achievement is to completely finish the game, so there are no mobsters attacking you, and you have all the businesses.
start off by getting them all in new york (if you have not already done so) because these are the easiest safes to get.
then move on to all the ones in florida, and finally all the ones in cuba. florida has the most so i would save that until last.
also there is a handy guide that ticks them of on the dons view, here are the directions, press start for dons view ---> press the button for "more dons view" ---> then move the blades across to "the books" then once on the books go on to safes. from there you can easily see where the safes are and if you have done them.
please comment on my guide if i helped! thanks :)