You can practice. You could imagine a shape like a cube, make it a certain color, and try to imagine it in one place for a while. Then turn it around so you can see all the sides, then try to see it as far away and different sizes. Then if you get really good at it you imagine other shapes, and then move on to more coplex objects. If this is really difficult you could also try a simple object, like a candle flame, that is sitting in front of you in real life. Then if the image fades in your mind you can open your eyes and see it again.