Put the observer beside you and slightly behind, on your non-casting-arm side, clear of the rod plane — and with the sun behind their shoulder.

Directly behind you the cast is nearly edge-on: the loop collapses into a line and the observer cannot see its width or the shape of the fly leg, which is what the demonstration is for. From the side, the nose, both legs and the loop shape are visible at once — and that is also where you stand when the student casts.

Then check the light the way the guide says to check a course: background first. The observer must not look into the sun, and the sun should light the line against a plain background, because dark trees and bright sky both swallow a fly line. If the light is wrong, move the pair of you, not just the observer.

The guide gives no fixed position; it gives the test one has to pass. A demonstration has to be easy to see, and the style must be something the student can understand, repeat and retain.

Source

The CI Study Guide (2025) (PDF) does not prescribe a position; this answer follows from its demonstration and course-setting guidance. Terms from the FFI Fly Casting Definitions.