Significance
Govardhan Puja, on the first lunar day of Kartika Shukla Paksha (the day after Diwali), celebrates Krishna's lifting of the Govardhan Hill on his little finger for seven days to shelter the people of Vrindavan from the wrath of Indra's torrential rains. This act established that the divine is present in nature itself (the hill, the cows, the land) and does not need elaborate rituals -- direct love and protection of nature is true devotion. It is also called Annakut (mountain of food), where enormous quantities of food are offered to Krishna and then distributed.
Preparation
Rituals & Vidhi
Govardhan Hill Creation
Make a symbolic Govardhan Hill using cow dung or mud, decorated with flowers. Place a small Krishna idol atop the hill. Some families make elaborate hills with multiple food items arranged around them.
Annakut Bhog
Offer a large variety of foods (Annakut) to Krishna -- the more the better, representing abundance. At temples, 56 items (Chhappan Bhog) are traditionally offered.
Mantra: Om Kleem Krishnaya Govindaya Gopijana Vallabhaya Swaha
Govardhan Parikrama
Circumambulate the symbolic hill (or the actual Govardhan in Mathura) clockwise seven times, remembering the seven days Krishna held the hill aloft.
Cow Worship
Worship the family cow if one is accessible, or visit a Goshala. Offer grass, jaggery, and garlands to cows. Feed a cow some of the Annakut prasad.
Family Guidance
Have children help build the small Govardhan hill from cow dung or clay and decorate it. Cook as many different dishes as possible as a family for the Annakut. Distribute the prasad generously to neighbors.