I have seen no illustration for this scene but I find no difficulty in imagining it.
We’re told that Caspian
... caused [the words] to be cut on a smooth cliff facing the bay ...
Clearly, then, one or more of the
Dawn Treader’s crew inscribed the words, with no need for magic, using ordinary tools which were already part of the ship’s equipment. We may safely imagine that each letter of the word was over a few centimeters in height (for otherwise it would be too difficult a task for presumably inexpert monumental masons to inscribe legibly) yet no larger than a handspan or so.
We may imagine that there was a sandy or rocky section of beach, with scree, or perhaps even a pleasant stretch of greensward, between the cliff and the bay because
they were all standing looking at the inscription.
We are not told how high the smooth cliff was but, assuming that it was twenty meters high, one can easily consider that it was so smooth, and, by definition, nearly vertical (and perhaps even looming over the ship’s complement) that it would have been virtually impossible for any but the most experienced rock climber to ascend even with the aid of ropes.
King Caspian, being a fit young man, could have easily tossed the ring vertically ten or so meters; the ring, at the vertex of the throw, when it had minimal acceleration, then landed “
on a little projection” of the otherwise sheer rock-face, and stayed thereon.
A particularly determined crewman, no doubt, could have gone to some trouble to gather some ship’s lines and to rappel to the ring but, since Eustace, Caspian and Lucy declined to keep it, we may safely assume that no one else really felt inclined to collect it.
So, one may imagine a section of sheer cliff of more than twenty meters in height, close to a bay, featuring an inscription, of as much as one square meter in size, with the top thereof being about one and a half meters above the fairly flat ground at the cliff’s base base; and, say, seven or eight meters above the inscription, a gleaming bracelet can just be discerned on a slight projection of the rock-face.