The Momento Mori is done in three parts: the roll of the dead from the Burning Times, the roll of the dead who were victims of other religious and political persecutions, and the roll of the dearly departed. First of all, a circle is cast so that all the participants may enter into a sacred and safe space for this deep cleansing of the spirit. This may be done however seems most appropriate to those doing it, and I know that the community in which I first experienced the ritual had no set way of casting it. Once everyone is settled (it is recommended that participants be seated on comfortable cushions on the floor so that they may move as they need to), a bowl is passed around with slips of paper. Everyone takes a slip until all have been distributed. These strips carry information about those who are confirmed to have died as a result of the Burning Times. Starting in the north, each person reads aloud what is on one strip. This continues clockwise around the circle until all strips have been read. Once those have been read, the priestess leading the ritual says (and may invite the rest to say with her) "we will not forget".
Next, the priestess invites people, to share the names of those people who have died from religious and political persecutions in modern times, whether it is a specific person or an anonymous group. This should continue until the priestess senses that the group is finishing. All say again, "we will not forget".
The final section of the ritual begins with the priestess inviting people to speak of their own beloved dead, not only names but as much information as they feel moved to share. This also continues until the priestess feels that the group is winding down. Once this section is finished, such chants as "Hecate, Cerridwen" may be sung to give closure. Try to end this section on a positive note via chants so that the catharsis provided by all three sections may be brought to a gentle conclusion, helping people return to a space where they can ground once the circle has been closed. Close the circle in whatever manner seems appropriate.
Afterwards, let folks sit together over non-alcoholic drinks (better for grounding) and munchies to share their feelings and center themselves. It can take a while to do this ritual and process the after-effects, so I recommend doing it on a weekend, so that most people can take their time and not have to worry about getting to sleep right away.
On a side note, I recommend from experience having many boxes of tissues available as most of these rituals involve large amounts of crying. It can be exhausting and draining, but it is also an affirming and cleansing ritual. I have always found that afterwards on a mental, emotional, and spiritual level I am much more clear and that it is good for getting rid of psychic junk.