When do I have to make my intention to fast?

There is no obligation to stop eating 15-20 minutes before Fajr. One may eat and drink until the actual entrance of true dawn.

The ruling returns to a broader principle: if a person deliberately causes something to enter his internal cavity (al-jawf), or any hollow space within the body, the fast is invalidated.

If a person performs iḥtiqān (administers an enema) into the rectum, his fast is invalidated because the substance reaches the internal body cavity (al-jawf).