Use the standard opening ritual, up to the Calling of the Kindred. Only call the Ancestors. Then continue, but after the praise offerings (which should all be offered to ancestors), the following is said:

It is said that the dead have a journey to make.
It is said the that dead cross a river on their journey.
It is said the the river becomes filled with their memories.
It is said that the river is filled with their wisdom.
It is said that the river comes to the surface as sacred springs.
It is said that those who drink from those springs
receive the wisdom and the memories of the dead.

A bowl of spring water is held up. Someone says:

From the ground it comes,
bubbling up with ancient knowledge.
When we drink it,
may we be filled with this knowledge.
When we drink it
may we commune with the ancestors.
When we drink it
May we meet with our beloved dead.

The bowl is passed around for all to drink. When all have drunk, light a candle from the fire, and say:

We light the way for our ancestors' return.
May they come and feast with us.

Process to where your feast will be, the carrier of the ancestor candle going first. At the feasting site, put a plate in front of the candle, and provide it with food. Later dispose of the food either in a cemetery or in water.

Note that I have left who says what open. The section with the "It is said"s can be done by a number of people, with each person saying a different line. (This will make memorization easy.) The part with "When we drink it" can be said as a litany, with a priest saying "When we drink it" and the others responding.

When the water is passed, it may be drunk from the common bowl, drunk from a dipped in hand, or drunk from a separate cup carried by each person, depending on grove practice.