I think I might have some useful advice, coming from my personal expirience. In the past few meditation sittings I have noticed that there are particular skillsets for exploring and adventuring within the mind.
When you want to meditate, or as I would say "explore the mind", you will first need to develope the ability to calm almost all obvious processes that are going on. It starts with thoughts and conceptualization, or what people call the "monkey mind". But it also encompasses the "virtualization of raw input", for example when you look at a chair, you should be able to make the object go away, and only leave the raw colour information that is present on your visual perception. All of this you can do by focusing on the moment, and really trying to let go of everything. This, on a good day, usually takes a few minutes for me. In general one could say that you need to be able free up space or psychic energy, to leave processing power for the tasks ahead. Conceptualization, dimensionalization and interpretation processes need all to be shut down to minimum to continue. 
 
The next skillsets are holding focus and directing focus. You need to be able to change focus on any perceptions at will, and you need to be able to hold your focus where ever you please, for however long you want. This is a difficult skill to develope, you can try yourself by focusing only on a point on a wall. It's not as easy as it sounds, because even the point can represent a multitude of perceptions within you. You might focus on the visual information or the position, even a certain feeling the point evokes in you, which are all very different thing. What the mind tends to do is switch around between these subtle perpections, of the same "object" so to speak, without really noticing that it is shifting focus. Focus tends to be very unsteady in the untrained mind. Learn to control this feature, because it is essential for further exploration purposes.
 
The even harder part, and why I think it takes years for people to become "enlightened", comes next. You might never, ever even come close to enlightenment if you do not have the skill of "finding new ways". It's hard to put into words, but the mind has the ability to find new things to focus on. New perceptions, so to speak. So for example, you might be able to focus greatly on the raw sound that you hear, but you might not be able to focus on the processing of sound that your mind does every single second it is operating. Every single sound you hear is brought in context to previous sounds, creating sensations of harmony or disharmony depending on the sounds. So, as everything is really happening in the moment, you will notice that there is a certain memory to sounds. This process you can become aware of, and you can focus on it. But you can only do that if you find the process. So it's essential to have the skill of becoming aware of processing within your mind, that you previously were not aware of.
It is one of the most important tools of exploration. When talking about consciousness, or the witnesser, you could find a hint of it today if you just found the right way to recognize it. For example, if you focus on visual perception, you can ask yourself "Who or what is seeing?". This question can be completely empty and mean absolutely nothing to someone, or it could trigger someone else's mind to go seek new "processes" that are taking place within their mind. So, the same words can mean nothing to one, but trigger a new way of "seeing" at things in another. This is a very particular skill, and you can notice that it is actually happening. It's like switching your "focus beam" at something you have never had your focus on before. And like this, you can explore your mind. Of course, it's quite hard to learn this skill, especially before you even know what it is.
To that skill I would say comes another difficulty, the difficulty of recognition. Focus can sometimes jump to one new "process" of your mind, be confused, and jump to another without really recognizing what it is. So, it is quite possible that you have already had an expirience of witnessing the witnessing, but you simply didn't recognize it. It's quite strange, but I would say it has to do with how much psychic energy you have free to use? I don't know.
And the last skill would be to be able to remember how to get somewhere. Exploring the mind is far different from exploring dimensionality. You cannot really remember the way, if you know what I mean. For example, an enlightenment expirience might have been trigger this way:
Focusing on visual perception -> focusing on the flatness of visual perception -> focusing on the "canvas" on which visual perception is happening -> focusing on what is actually expiriencing the canvas, or where it canvas is happening
Now, the last step would be the focusing awareness on itself. There are two problems though. If you haven't developed the ability to hold your focus, you might just "expirience" awareness for a split second and then your focus will crumble and fall onto something else once more. The far bigger problem though is remembering how the hell you got there. Sure, you can mark each steps with some conceptualizations, but that will not really help you to get there again. It's hard to explain, but I think people who do consciousness work will know what I mean. Our mind has not really a basic function for remembering the "location" of processes. It's not really a location anyways. The mind is really bad at creating "mind maps", and it's even worse at following the whatever maps it created.
So in the end you really have to do it so many times, until it becomes "muscle memory". I do not know if there is another way but intuition. This means the first enlightenment expirience might be years apart from your next, but the more expiriences you have, and the more often you "went the way", you basicly get better at it. I guess there is a genetical disposition to people who can intuitively do this faster than others. Like some people can simply learn faster to paint, some might learn faster to become enlightened.
 
So basicly, these are all the skills that, as far as I am aware of, need to be trained:
1. Clearing and shutting down mind processes
2. Steering Focus
3. Holding Focus
4. Finding new "mind processes" to focus on
5. Recognizing new "mind processes"
6. Remembering how to move your focus to specific "mind processes"