The Black candle represents the Void of Death, it represents the night, the left hand of witchcraft and necromancy. It also receives, it receives the wisdom of the dead, the power they hold, the strength and ability. The white candle is our projection candle, where we use it for prayers and offerings as white sends out all other colours.
The two, side by side, also represent a sort of gateway and are a minor representation of the Macrocosmic Existence. The skull or image of death is placed in the center to be a focus, a gateway between the two pillars of Life, Death, Mercy, Severity, Wisdom, Understanding and so on. The red cloth, if used, represents the life blood and represents thus the duality of life and death, death and life, the final breath and the first one and is thus a very good meditative quality for the altar. The glass of water is placed on the left, to receive, the incense can either be placed before the image of death or on the right to send forward prayers to spirit world through the incense. The plate and bowl and other such items are used as offering dishes and are generally kept near the altar for ease of access. The mirror is also kept near the altar but is wrapped in a cloth and kept under it when not in use. Additional items include bones, a wand of cypress, willow or yew, keys of any sort, shot glasses for our Darling Spirits that like alcohol, replacement candles, engraving tools, intention candles for the occasional spell, divinatory devices, holy water or salt in case something needs to be kicked out, papers and needles, pens and pencils, photographs of the dead such as ancestors or even friends who we know care for us on the other side. If you are able to and have made your pet a familiar (after their death, so you have their bones or their photos), that's another good idea. If you have any other animal bones with spirits that have agreed to work with you, that's good too. Pigeon feathers are also good, pigeons have been used as messengers. A stang is also a good idea for when one wishes to walk between the realms. Herbs such as coriander (which summons spirits), jasmine (which summons wise and helpful spirits), camphor (which keeps out bad spirits and aids one in cleansing), copal (which is good for exorcisms and exalting the dead), myrrh, frankincense and roses are also good. One may also choose to keep an intermediary spirit on the altar. This can be a familiar spirit, a demon, a saint, an angel and so on. It may also be a good idea to keep a separate notebook for the dead, to keep track of their offerings, holy days, what they like, gnosis, pacts etc.