Since I am my own artist, I just put enough in the description to have a basic understand of what to draw and worry about details when I am working on the art.
My scripts usually look like this:
Pg 1
Panel 1.1 Description of art
Character 1: Dialogue
Panel 1.2
Description of art
Character 2: Dialogue
Character 1 : (thinking) Thoughts
Panel 1.3
Description
Character 1 (Off-panel): Dialogue
Character 2: Dialogue