How do you decide how the characters are going to look?
It usually starts with their histories – who they are, where they come from, what role their ethnicity plays on it (if any). For example, Bryn's appearance in the Kanin Chronicles is determined by her mixed heritage, and I decided on the various troll tribes appearance based on what their abilities are and what I thought they would be doing with their life.
For characters where that's not an issue, like in the Watersong series, it's usually based on what I think suits the character. Gemma was more beautiful than Harper, so I gave Harper wilder hair. Gemma's younger, so I made her a bit shorter. For Daniel and Alex, I just based them on what I find attractive, since they're the romantic leads.