Is anyone able to make the imaginary imagery appear in the primary context, right before their very eyes, replacing whatever the current input may be?
This is how I went from not being aware "picture yourself on a beach" was more than a figure of speech to being able to decompose mechanisms and take measurements all in my mind.
This is exactly how you do it.
Your eyes have nothing to do with it. Your brain fakes your eyes seeing stuff for you, and it can fake other things for you if you ask it.
I can decide not to process what comes in my eye, as long as it's not a tiger or something. Then I don't see it. If it's a familiar face or a threat, then it's harder to phase out, almost impossible.
It's useful to have something better to fidget with than a black square. But I can do black square.
Also your actual vision is like a spot, not like a square.
I never have trouble imagining things as they do clearly 'render'—I can picture scenes, shapes, faces, motion, the styling of text I remember, basically anything—but it doesn't result in true visual output where I can see things as if I have my eyes open. If the latter is something that people can truly do, then I guess I do have some form of aphantasia.
Focus on a point. Move that focus around.
Get to two points. Spread them apart, move them closer together.
Advance to tracing letters. One after the other, into a word.
Movement is key. Try reading that backwards.
And so on.
Myself, it took about six months from nothing to colored triangle.