Find 5 objects with simple structures and low amount of details (ex. Mug, apple, tablespoon measure, baseball, salt shaker). Now look at one of the objects and study it, look at every detail and do this until you think you have it in your mind. Now close your eyes and imagine the object. If you're having troubles, look at it again and see if you can remember more details. Imagine it again with eyes closed and repeat looking at it and imagining it until you feel you have the object fully remembered visually. Do this exercise with the other 4 objects you chose.

Another way is to simply imagine a room in your house. All the details, carpet color, walls, that shoe in the corner, TVs, whatever is in that room. This skill is pretty useful when you're trying to expect something while in a lucid dream.

Personally, I don't visualize visually when I'm wilding, I'm more of a kinetic and sound person. But visually visualizing is extremely useful when you're controlling dreams and trying to manifest something.