One who is required to eat on Yom Kippur due to medical reasons, should not attempt to fast against the doctor's orders. The same Torah which commands us to fast on Yom Kippur tells us that guarding our health is far more important than fasting on this holy day. Just as a healthy person fulfills a mitzvah by fasting; an ill person does a mitzvah by eating.
A rabbi should be consulted in advance to receive specific dispensation from fasting. However, in case of a medical emergency, or even if there is any question whether the condition is a medical emergency or not, the patient should be given food or medicine immediately.
A person who is required to eat or drink due to illness, but afterwards feels strong enough to fast, must resume fasting.
One who is weak and can only fast if he remains in bed all day, should not attempt to go to synagogue or pray, rather he should fast and remain in bed. Attending synagogue and/or praying are of secondary importance in comparison to the biblical obligation to fast.
According to the Torah, one is only "culpable" when eating at least 1.26 ounces of food, or drinking at least a mouthful of liquid, within a short period of time. If possible, the ill person who must break the fast should eat and drink less than the abovementioned amount at intermittent intervals. Preferably, one should wait nine minutes between these snacks. Obviously, if the medical condition does not allow for this intermittent snacking, the patient must follow the doctor's orders.
In an ancient Machzor (High Holiday prayerbook), it is suggested for an ill person to recite the following prayer before partaking of food on Yom Kippur:
Behold I am prepared to fulfill the mitzvah of eating and drinking on Yom Kippur, as You have written in Your Torah: "You shall observe My statutes and My ordinances, which a man shall do and live by them. I am the L-rd." In the merit of fulfilling this mitzvah, seal me, and all the ill of Your nation Israel, for a complete recovery. May I merit next Yom Kippur to once again fulfill [the mitzvah of] "you shall afflict yourselves [on Yom Kippur]."May this be Your will. Amen.
When one is required to eat, no Kiddush is recited on Yom Kippur. (This applies even when Yom Kippur falls on Shabbat.)
Before eating bread one must wash hands as is done throughout the year. There is no need to have two complete challahs, as is required on all other festivals.
There is a difference of opinion whether the holiday ya'aleh viyavo is reciting during the Grace after Meals.