(Please consult with your own marja for further clarification on rulings.)
The following is based on the Opinion of Al-Sayyid Ali Sistani
NIYYAT FOR FASTING.Fasting means that you must, in obedience to the commands of Allah, from the time of Adhan for Fajr prayers up to the time of Adhan for Maghrib prayers,
It is not necessary for you to make the Niyyah for fasting through your mind or to say that I would be fasting on the following day. It is sufficient for you to decide that in obedience to the command of Allah I will not perform from the time of Adhan for Fajr prayers up to the time of Adhan for Maghrib prayers, any act which will invalidate the fast.
In order to ensure that you will be fasting throughout this time you should begin
your fast a little earlier than the time of Adhan for Fajr prayers, and break your fast a little later after the time of Adhanfor Maghrib prayers.
You can make Niyyah every night of the holy month of Ramadan that you will be
It is important to remember that the last time for making Niyyah to observe a
It is very important to keep in mind that your Niyyah of fasting should remain constant throughout the period of fasting.
If you are undecided in your Niyyah whether to break the fast or not, immediately your fast becomes invalid even if you do not actually break it or are repentant of your intention.
THINGS WHICH WILL MAKE YOUR FAST BATIL
There are eight acts which if you do will make your Fast Batil.