The German magicians Rah Omir Quintscher (1893-1945) and his well known Czech companion Franz Bardon (1909-1958) first described the Soul Mirror as a quintessential technique of magical introspection.
"This self-analysis is one of the most important preparatory works in magic. Many occult systems neglect it and thus will not have much success. This spiritual groundwork is most essential to the magical balance, without which there is no regular progression in development." (Franz Bardon)
The idea of the original Soul Mirror - as the name suggests - is to take a ruthless look into the mirror of our soul. To create an inventory of all qualities present within us, both dark and light, loved ones and shameful ones. To break through the thin veils or thick walls of narcissism and take stock of what we are truly made of. If we were practical alchemists this was the moment when we inspected each plant in our gardens and every flask in our laboratory - just to understand the raw materials we were given by nature and our ancestors to approach the Great Work. The more honest and unclouded, the more calm yet merciless, the more caring yet coldly observing our view can be, the better all work will turn out that we will undertake from here onwards.
The result of a glance into the Soul Mirror shouldn't be a state of self-flagellation or false subserviency. It takes a healthy dose of self-confidence to get this right. You are still 'okay' despite all the undesirable qualities you found within yourself. It simply is the premise of any successful magical work to look beyond one's limited subjective perception of the world.
Here are the practical steps to create your a complete Soul Mirror of the Elements. Feel free to skip steps depending on your existing previous knowledge or experience in introspection.
The most advanced version of the Soul Mirror of the Elements can be found in Stejnar's excellent book pointed out above. Not only does it take into account the four elements themselves, but also the four primordial qualities that create these. For any German readers, I strongly recommend to read this book cover to cover.
However, to give you a first starting point on how to approach the second phase of the Soul Mirror let me share a few examples of mapping of inner qualities to the four elements below. It is important to remember that there is no objective key for this mapping. Our subjective experience of these traits and their impact on our personal and social lives is crucial to create a personally meaningful and coherent Soul Mirror that can be worked with practically.