Here's my take on it:
Take a couple of sheets of card stock (the cardboard stiffeners included in reams of copy paper are good for this)
Draw a circular target on one.
cut the other into two sights, one front and one rear.
Front sight an inch wide and 2" tall with the rest of the card forming a handle so you can move it around easily.
The rear sight 1.5" wide and 1.5" deep, also with a handle for moving it around easily. The two handles should be oriented so they don't interfere with one another.
Lay the target on a table and "acquire" the sight picture as you would any firearm, putting the front sight on target and the rear sight centered (following the generally accepted rule of keeping focused on the front sight).
Have your student demonstrate the same thing until it becomes second nature.
Note that all this visualization/practice is with the 3 pieces of cardboard in the same plane.
Then, separate the target from the sights' plane by about a foot. Then as understanding increases, separate the front and rear sights.
Then, start moving the target around while the student "tracks" with the sights.
Now, mount the "sights" on a ruler or yardstick, essentially forming a paper and wood firearm. You can watch (evaluate and critique) the student manipulate the "firearm" easily because your eye is right there with (or as) the target.
Lost Sheep