When you see the new moon, observe fast, and when you see it (again) then break it, and if the sky is cloudy for you, then calculate it.

سَمِعْتُ رَسُولَ اللَّهِ ﷺ يَقُولُ إِذَا رَأَيْتُمُوهُ فَصُومُوا وَإِذَا رَأَيْتُمُوهُ فَأَفْطِرُوا فَإِنْ غُمَّ عَلَيْكُمْ فَاقْدِرُوا لَهُ