To set an ancestor altar, it doesn't need to be fancy, elaborate, or complicated. A simple white cloth over a table, their images, glass of water or coffee, white candle, flowers, and something that they enjoyed while living. If you are uncertain of what they had as a favorite food, cook a popular dish from your cultural ancestory, or national dish of the country they were born. Sit in front of their altar and talk to them, pray to them, remember the connection you once had, or if you didn't know them, the connection they may have had with other members of your family. Once you feel the connection ask them for what you are in need of.
Many times spiritual workers leave the altar up year round, although it isn't necessary to keep this altar up all the time, you can set it up for making a specific request then break it back down in a few days when the candle is finished if you are using a 7 day vigil. Some choose a day of the week to make offerings to their ancestors. I personally choose special times of the year, holidays, their birthday, and in times of needing some guidance as the days that I leave offerings. Your ancestors are with you everyday. Take the time to make offerings to them. Ancestor work is a very important part of the work with traditional systems such as Vodou. So I suggest that if you are thinking of Vodou, honor your ancestors first most.