My impression is that it seems to involve a very detailed description of the consequences of a particular trait or action, such as anger. The "therapist" paints such a detailed word picture of the consequences that the "patient" learns from this as though they actually experienced it.
Am I inadvertently quoting this from a source posted above? Anyway, the extant sources are limited so "setting before the eyes" could involve more than just this, however this seems to be all that's in the sources.
This brings to mind an experience I had as a teenager: I learned to waterski by sitting in a room with a friend and listening as he talked me through each step of the process, from getting into the water to standing up. He did this on two or three occasions. His explanations were so vivid and detailed that the first time I got in the water I was able to ski as if I'd been doing it for quite a while, and continued to progress from there. I've always been amazed at how that worked! I consider that to be "setting before the eyes", although not in the Epicurean context.