Part 1 and Part 2 can be found here. This section focuses on ancestral work. This connection to the roots and the lineage is key to further progress in spirit work. Ancestral veneration is important in every culture and religion: these lessons can be applied to any spiritual path which calls you.
In Vodou the dead reside anba dlo, en bas de l'eau, under the water. Deceased members of a société (a Vodou house) may be reclaimed later and housed in govis, ceremonial clay pots which show they have been drawn back to our world like water drawn from an ever-flowing river. Water gives us life and keeps our dead; it is the home to Les Ville aux Camps (Milokan or Minokan in Kreyol) where the lwa reside, the great barrier which separates us from and carries us to Gineh.
Because of this, water can serve as a spiritual gateway. The offerings, holy symbols and images help them to feel at home and provide grounding, while the illuminations guide them. The glasses of water which you place on your altar are the doorway through which your beloved dead can return. By staring into that water, you may be able to speak to your ancestors through scrying. Focus your eyes on a point within the glass and relax: make note of any images that rise within your field of vision, but do not try to control the visions or force the action.
(As with everything else, this improves with practice. Scrying is also a bit of an innate talent, so if you have little luck don't despair: there are many other means by which you can communicate with your ancestors and I will be discussing them in due course).
Water flows into all crevices and fits all containers: it takes unto itself the nature of all it passes over and through and returns it to the great mother ocean. You can use this to your advantage: water drawn from a stream or river in your family's ancestral homeland, for example, can be used to establish your ties with the oldest roots of your lineage. Put this in a small vial or bottle which you then seal with wax (so as to avoid exposure to the air and other elements), and place it on your altar. Like a river, it will provide a means of transport between distant places and times.
Because it takes all things into itself, water can purify an area. It is vital that you change the water on your ancestral altar regularly: I would recommend doing so each evening before going to bed. Fresh water attracts positive spirits, but standing water which has become stagnant attracts spirits of decay and corruption. On an aesthetic note, it also becomes covered with scum and the bodies of small flying insects -- hardly the sort of thing one wants on an altar.
Do not confuse water with alcoholic spirits, particularly hard liquor. Water cools the spirits and makes them more peaceful: alcohol heats them up. You can certainly give alcoholic beverages to an ancestor who enjoyed a drink in life, but those drinks should be placed on the altar along with water, not as a substitute. (And you may wish to refrain from giving such offerings altogether if your family has a history of alcoholism).