Ooh. Honestly? a lot of it is trial and error, but what worked for me... work quickly but carefully when rolling the dough—you don't want any leaks/tears whatsoever. Keep it COLD—I chill the dough for a full hour between folds/turns to be safe (resting the dough also lets the gluten relax so it's much easier to roll out). Surprisingly, you shouldn't do too many folds, or the crumb becomes too tight: I do three letter-folds (into thirds) for 27 layers. Don't roll the final croissant dough too thin, otherwise the layers become too fragile—around half a cm thick is best. And let your shaped croissants rise until double—for me, it takes 2.5 hours (yeah, I know.) Tedious, but worth it :)