Here is a description of shimasana (lion pose) from Vanda Scaravelli's book "Awakening the spine" (Vanda was a pupil of Mr Iyengar in her early "yoga years"). Perhaps that helps you a little.

Find a comfortable sitting position.

"In the lion pose the mouth opens completely and the tongue is exposed in all its length and breadth. It also gives the throat and the lower part of the neck, which cannot otherwise be actively involved, the chance to open.

Inhale with your mouth closed. Exhale with your mouth wide open, throwing your tongue totally out and spreading it in all its width by unfolding the edges and, at the same time, pushing it out and down, as if you wanted to touch the chin with it.

You will feel the extension in your muscles, not only along your throat and neck, but even behind your ears, in the same way was when you yawn.

When you have finished your breathing exercises lie down in Savasana, even if you only for a few minutes, completly relaxed, breathing naturally and abandoning your body totally to the earth."

I'm sorry I don't have the Iyengar's book "Light on Yoga". For further instruction perhaps you can buy it or visit your local library and see if they have it.

All the best, Rita