A Method of Self Guided Mental Imagery
Firstly, define the aim of practicing mental imagery.  For example, you might have some difficult movements that you wish to improve.  Or perhaps your aim is to improve the overall level of your Tai Chi, the flow, speed, the power of your movements or even to reach total integration of body, mind and spirit.

It is a good idea to set a long-term goal, eg. In three years you want to reach the target that 80% of the time when you practice your body, mind and spirit are integrated.  You then need to break it down to many steps for short-term goals, eg. in one month’s time you will achieve the body being upright.  Having these short and long term goals in mind, plan how often you will practice mental imagery.  

The advantage of mental imagery is that it works on the unconscious mind, therefore it can be effective in a way that sometimes a lot of physical practice alone cannot reach.  In another word whilst the physical practice is very important, the mental practice can help just to reach that bit further which you will not reach by physical practice alone. A reasonable time ratio of 10% to 20% of your practice time could be devoted to mental practice.  

Having done that, you should allocate a dedicated time, maybe 5 to 10 minutes to commence mental imagery practice.  Try to imagine everything as real as possible.  Go to your normal practice area if you can, feel the atmosphere there, feel every little movement, put yourself in a situation where you are not likely to be disturbed within that practice time.  

Now work on your goal.  If you are working on your body being upright, your 10 minutes will consist of you seeing yourself performing a set of forms from beginning to the end, preferably in real time, every detailed move, you see yourself and think of yourself performing the move with your body maintaining upright.  Whenever you notice any leaning forward or backward, mentally correct yourself. In your imaginary world you are powerful, you can do anything. Here is the advantage of mental imagery, you can imagine yourself as perfect as you want to be and because the unconscious mind does not know whether it is a real or an imaginary situation.  Your unconscious mind has imprinted this image of you being perfect in motion.  As we discussed, our unconscious mind has a powerful influence in what we can do and if you now have a strong imprint of you being perfectly upright, you will.

Practicing Tai Chi we should be tranquil and in a high spirit, with confidence and mental comfort.  Most practitioners realize that acquiring this feeling is difficult, although it will help to improve our level of Tai Chi.  Using these training methods we can train ourselves to have this tranquility, clarity of mind, and positive high spirit.

Conclusion
There are different ways to look at the internal and external.  In reality these components are inseparable.  It helps to focus on one aspect for a length of time in order to improve that aspect.  That is why something we need to define and separate them, after improving all aspects we can then put the bits together to reach a higher level.

I must emphasize that some of things in the internal aspects might sound easy, but they can take a lifetime to reach a high level. You will need to be patient. Remember that there is no perfection in Tai Chi but you will enjoy the process of getting there.