A Festival of Isis involves standing in a circle of seven decorative torches, applying a perfume, and sacrificing an ambrosia of flavored honey and oil at a standard altar.
The decorative torches will be one of three possible colors. Some altars throughout Egypt have already been equipped with seven torches of each color, and are listed on the Festival Altars page.
The perfume must be made of a randomly determined citrus fruit. Perfume takes 10 minutes to make at a kitchen by a person who has the Perfuming skill, and requires 10 [citrus fruit]? of whatever type you need to create. Note that simple perfumes last only a few minutes so don't apply them until you begin the ceremony.
Ambrosia is also made at a kitchen with the Ambrosia Brewing skill. Isis ambrosias require 1 oil and 1[flavored honey]?, which (along with the citrus fruits used for the perfume) are made by an Indonesian bee hive. Only players who have passed the first round of competition in The Test of Khefre's Children can learn the Indonesian Bee Care skill needed to make and operate these, so you'll have to track one of them down to find your type of honey. Torches require 10 oil to light. Thus to light the 7 torches a player needs would require 70 oil, or if you are preparing an altar for a group of people and need all three kinds lit (21 torches) you need 210 oil.
A list of people supplying various Isis supplies is here.
See also: The Test of Festivals, Determining Festival Requirements, Festival Schedule, Festival Needs, Festival Altars.