The greatest mistake that tai chi students can make is believing that coming to the tai chi school and learning tai chi movements is the essence of tai chi.

 To identify tai chi with the movements is to lose the meaning of tai chi and reduce it to a sport.
The tai chi movements are merely aids in helping practitioners to root themselves in their wu chi nature so that tai chi can become possible.

(John Lash)

If your practice of tai chi does not embody the principles of tai chi, then it is not tai chi.

This is not a matter of opinion or of right and wrong.
Consider: karate is not tai chi because it approaches combat and body usage in a fundamentally different fashion to tai chi.
Karate is karate, just as a tree is a tree or a car is a car.

These qualities should not be present in any facet of your training:

1. contracted muscles/tension
2. locked joints
3. strenuous postures
4. independent limb movement
5. gaps and deficiencies of structure/flaccidity/no connection
6. no groundpath
7. strength versus strength
8. dependency upon speed

Tai chi can only be tai chi if it reflects the nature, the essence, the way of tai chi.

For your practice to be uniquely tai chi, it must address the Tai Chi Classics, 13 postures, jing, 4 ounces of pressure, softness and yielding.
If it does not, you are not learning tai chi.
These aspects represent the centre of the training - the what of tai chi.Neigong is the how.

Once these components exist, there are many different ways you can implement them.
Every style and school of tai chi has its own approach.
Providing the Master follows the key principles, this diversity is healthy and natural.
No one is right and no one is wrong - just different.

Taoism examines the nature of things; exploring what characteristics make a thing unique.
What makes a tree a tree, rather than a dog?

When practicing tai chi, it is important to be aware of the qualities that make your exercise tai chi, rather than karate.
Ask yourself: what is the essence of tai chi?

If you cannot answer this, you may need to do some serious research.

Water demonstrates the yielding principle perfectly.
Its very nature is yielding.
From this observation comes the principle 'wu wei' - commonly seen as 'going with the flow'.
Wu wei also means not forcing.

Water is the very softest of things, yet with mass and momentum, it becomes devastating without losing the softness.

Although it might seem that the necessary time to master the requisite skills and attain a level of expertise would depend upon the field and your own talent level, those who have researched the subject repeatedly come up with the number of 10,000 hours. This seems to be the right amount of quality practice time that is needed for someone to reach a high level of skill and it applies to composers, chess players, writers, and athletes, among others.

Although the number of hours seems high, it generally adds up to seven to ten years of sustained, sold practice - roughly the period of a traditional apprenticeship.
(Robert Greene) 

Most tai chi students are content to attend classes, enjoy themselves and make leisurely progress through the syllabus.
They are not overly ambitious.
Some students are different.
They seek to acquire the inner teachings, become an indoor student, a lineage disciple and truly steal their Master's art.
These students are essentially undertaking a tai chi apprenticeship; a lengthy journey of study and practice.

Tai chi encourages students to look at life differently. This is not a political or religious agenda.
It is a realisation that how we see the world has been shaped by our culture, upbringing and education.
We do not see the world as it is.
Consequently, we live in a fragmented, conflictive manner.

This is not the tai chi way.

Seeing reality without conditioning is not necessarily possible once you have been conditioned.

It is best to start simply - by learning to be aware of your own body and its relationship with the world around you.
In tai chi this is accomplished through solo and partner exercises designed to increase your physical sensitivity.
Later, more challenging exercises explore how your mind responds to a variety of situations and how the need for security affects your choices.

The real work is beyond the class.
With your changing perception, you begin to re-consider the way in which you live.

Recommended reading:

1. Mastery by Robert Greene
2. Steal My Art by Stuart Alve Olson 
3. Moving Towards Stillness by Dave Lowry
4. There are No Secrets by Wolfe Lowenthal
5. Zen in the Art of Archery by Eugen Herrigel
6. The Book of Five Rings by Miyamoto Musashi
7. The Power of Internal Martial Arts by Bruce Kumar Frantzis
8. The Sword Polisher's Record: The Way of Kung Fu by Adam Hsu

Most martial arts expect a student to reach a 3rd dan black belt standard before training to become an Instructor.
This is a good guideline for tai chi chuan also.

Typically a Tai Chi Chuan Instructor has at least 5 years experience and is ideally a lineage disciple.
They have a serious commitment to home practice and clear evidence of ability in all 13 areas of skill.
The Instructor should possess a stage 2 knowledge of form and be capable of teaching different forms e.g. Yang Cheng Fu, sabre, cane, staff, walking stick, jian (or equivalents).
They should be able to apply every posture of the Yang Cheng Fu form in a wide variety of ways.

After some years of practice your Master may ask you if you potentially want to become an Instructor.
The training will commence with helping out in class.
Simple chores and assistance.

If you show enthusiasm and competence, your Master may commence teacher training with you.
This formal process of learning will take many months, probably even years.
Once qualified, an Instructor should seek Instructor affiliation and insurance with The Tai Chi Union for Great Britain or equivalent body.