How to illustrate a children's book?

-I'm 15 and my cousin has hired me to illustrate her children's book she's always wanted to make. She will write the story and I will be the illustrator.

I'm not asking how to get my book published, how to draw, or how to write the story. She will be taking care of publishing and writing, and I'm already a good artist.

I'm asking literally how I should illustrate this book. I don't have any fancy digital art stuff, it will be drawn traditionally with colored pencils and watercolor. What size paper? What kind?

I remember way back in Elementary school when an illustrator visited and showed us her work. I remember seeing that you have to have a certain amount of space between the pages, where the pages are binded, and having to leave space for text and stuff like that. How much space should be around the edges? Any good tips and specifics I wouldn't know are very much appreciated! Thank you!!!