I have 4 different altars (no, five) in my living space (they kinda takeover the house after a while). They are peopled with altar clothes of appropriate colours, found objects, and candles and other items bought from botanicas or religious supply shops. I suppose the key items are 1 or more images to the deities involved, pictures of statuettes. Then just fill it with things that they like and then make offerings, light candles and incense on a regular basis.
A friend of mine used to have 7 altars stacked vertical in a 7 shelf Ikea unit. A heavenly transcendent deity was at the top, with the altar to the God of the Dead at it's base. We used to refer to it as "the Ikea Tower of Voodoo".