Unconscious learning is also something you have a great deal of experience with, but it's much harder to notice, and you may not realize that you do. One of the best examples involves learning to play a sport. Consider a ball sport, such as baseball or tennis. The way to learn to hit the ball is to go out and practice hitting the ball. As you practice, you start being able to hit the ball more often. You don't necessarily understand the details of what's going on - you're not studying computer models and saying to yourself "hm, I should start my swing 3ms sooner for an 2.7% increase in hit rate". You just go out and hit the ball, and you get better at hitting the ball. That's unconscious learning.
You learn things both consciously and unconsciously on a regular basis. In addition to initially learning things in different ways, however, it is possible for something you used to do consciously to become unconscious, and vice versa, and that's what the rest of this section is about.
What causes something which was previously conscious to become unconscious?
When you do the same thing repeatedly with little variation, your unconscious will learn how to do it automatically. You'll then be able to execute the behavior as a block, without needing to think about each individual step. This is usually desirable, as then the entire behavior can then be used as a building block - one of the steps in a more complex procedure.
Note that the phrase "with little variaton" is key. If you always respond in the same way to the same situation, you'll start responding in that way automatically. If you sometimes respond one way and sometimes respond another way, however, this behavior will stay conscious. It has to, because there's a decision that has to be made part way through, and thus the entire procedure can't be collapsed into one step (unless, of course, the decision is simple enough that the unconscious can learn to make it as part of the procedure).
For example, the unconscious is perfectly capable of learning one behavior for when the ball is approaching you from the left and another from when it's approaching you from the right, and executing the correct one automatically. This is not the sort of decision which would require the procedure to remain conscious.
For an opposite example, suppose that you select food in a restaurant based on a complicated function of what you think or know is good at that particular restaurant, what you've eaten a lot of lately, what you feel in the mood for, how hungry you are, what you expect to be eating in the near future, etc. Your unconscious probably won't learn to make this decision for you, and the process of ordering food will continue to be a conscious process, no matter how many times you do it.
If you want to make something unconscious, all you have to is to practice it, and execute it the same way every time.
What causes something which was previously unconscious to become conscious?
Unconscious activities are characterized by being a single step - an action which is executed without awareness of any intermediate steps between start and finish. This property can be used to make the action conscious by interrupting it part way through, and then failing to complete it or by completing it differently.
For example, consider walking. Suppose that you take a step, but before you complete it, you change the way you're putting your front foot down, and touch down your toes first instead of your heel. It won't be an automatic action anymore. You'll be aware of the entire motion, from the lifting of the foot to the lowering of it. You can go try this right now if you want.
Of course, you're not limited to interrupting physical actions. You can also interrupt emotional responses to situations, such as anger or hopelessness, and then either continue the response consciously or do something else instead.
Almost all actions are built on simpler actions. For example, you learn to see shapes, and then you learn to infer objects from these shapes, and then you learn to model the space these objects occupy and where they are in relation to yourself, and then you use this to walk without hitting them.
When you convert a behavior from unconscious to conscious execution, therefore, it doesn't become fully conscious - it's still based on building blocks which are unconscious. You've just removed one level of abstraction. Similarly, when you convert an action from conscious to unconscious execution, you've added one level of abstraction, and your conscious is now free to work with more complex concepts and behaviors. Your conscious is almost always active doing something; as you convert behaviors between conscious and unconscious execution, you're moving your conscious awareness up or down a level.