Hi!
this largely depends on the kind of space you have available and if you wish to make it a somewhat permanent altar or not. Generally speaking, the arrangement can range from the most basic solution (a silver/silvery bowl filled with water or a mirror to represent the Lake which is the body of the goddess) to a more elaborate one where you have the bowl/mirror in the middle, surrounded by four items that can represent the guardians (suggestions are: a goat’s horn or hoof for Faun, a figurine/statuette/painting of a wolf (or even an ethically sourced wolf bone/skull); same thing for the stag (figurine/skull/antlers) with the addition of a quartz crystal as an additional option; a clear glass filled with fresh water and a small gold item (like a pendant or an earring) for Egeria.
I generally use a black, white or red cloth depending on the type of aspect I’m working with but you could choose to always use a simple white one.