Visualization is primarily to WILD or MILD. It's hard to rank how well your skill is. The clarity of the image is a good indicator as always. When you get into that deep trance when the HI begins to form, thats when the visualization really kicks in. Your skill can be measured in how well you can transform and exert a level of control over the HI, and how clear the images are.

You can improve this skill through simple practice. I actually, to work on it, would through the day take 15 minute resting periods. In which I would lye down and close my eyes with no actual intention to WILD or sleep. I would practice my relaxation techniques and then once the HI started coming around I would practice manipulation of it to increase my visualization skills. I found doing these exercises periodically really helped "work out" that portion of my brain so I had better control.