Close your eyes and try to visualize an apple. Do this for 30 seconds or so. Try to visualize the skin, the reflection, the texture, the stem, the depth, etc. Try to hold a stable mental picture of that apple.
After the 30 seconds, rate your ability to picture the apple from 1 to 5, where 1 is complete inability and 5 is as if you were looking at a picture of an apple for those 30 seconds. 1 is aphantasia.
Another idea is to recall a vivid dream you had. I think most people would describe it as being part of a movie or reality. While awake, are you able to recreate scenes in vivid detail as if you were dreaming? 5 for complete parity and 1 for not at all. 1 is aphantasia.
https://www.youtube.com/watch?v=qvdVBzuGWr4
You can easily understand where the difference is because the data is different between the eyes. The difference appears 'ghostly'. In a similar way, data from the mind's eye is different from data from the physical eyes when those two 'streams of data' are blended.
Anyways, this is nothing like what I experience when I imagine something.
your imagination is more like it's in the the back of the head, yeah?
What helped me 'move' where my imagination was (to the front and center), was to do the flame meditation. Which is to focus on a flame in a dark room for a few seconds, close your eyes, and try to retain the phosphene afterglow in the flame shape. and repeating that until you are able to retain image of the flame while your eyes are closed.
Similarly: 'drawing from memory' - particularly from recent short term memory - was another method that had a profound impact on my ability to visualize.
Both of these take time and commitment, but they have worked for me. They may work for you.