Once you have the aliens clear in your own mind, you have to make them come alive to your readers. The goal is to evoke a sense of the alien in the mind of the reader. Physical description is one way; you can also talk about emotions, and sensations. You can use hints and suggestions. Or use dialogue.
Here are some examples, from published fiction, of ways to evoke the alien. (For illustrative purposes, I have condensed some of these passages. I encourage you to seek out these works and read them in their entirety.)
The Martian Chronicles: "Night Meeting"
(Poetic—using language to convey a sense of eeriness)
"Neutron Star"
(Literal description, including explanation of function)
Metaphase
(Another example of description by function)
Starsilk
(A sense of the alien, with words and thought, and no physical description at all)
Neptune Crossing
(A sense of the alien, conveyed through sound)
Dragons in the Stars
(The alien revealed through interaction with a human)
Harry Potter and the Chamber of Secrets
(Strange creatures revealed through humor)
