Guide to self-typing: the basics

Debunking some stereotypes

Being an extrovert does not mean you have to be a party animal or a popular individual with lots of acquaintances, wide social circles and busy days. While it is generally true that extroverts are more comfortable than introverts with such a lifestyle, there are also many extroverts enjoying their alone time and preferring smaller groups or one-on-one interactions.
And yes, extroverts can be shy too -- shyness or social anxiety does not mean introversion.
Not all introverts are socially awkward, nor they spend their whole lifetime living as hermits. They just tend to let others take the initiative, but some of them are not afraid of making the first step if they're really interested in someone.
They also value their alone time as a source of energy and relief: too many interactions with people or the external world can be very draining for an introvert.
Despite all the memes out there, perceivers are not "procrastinators". Everyone can procrastinate to some extent, especially in moments of stress, loops or grips -- this is a very common trait in introverted judgers, for example.
Being flexible and open to new opportunities -- in extreme cases -- can lead to commitment issues, but there are many perceivers who are very loyal to the causes or people that catch their interest.
And here we have the reverse of the medal: not all judgers are always neat, punctual and organized, nor they feel the need to overplan every aspect of their lives. They just tend to prefer some kind of structure and prior planning, but can be messy sometimes -- "organized mess" is really a thing, for some of them.

The basics of E/I and J/P

Extroversion gains energy from the external world of objects, that means interacting with the environment and other people. By rule, extroverts lead with an extroverted function.
Introversion is more inwardly focused and tends to collect informations primarily from the world of thoughts, feelings and the unconscious mind. Introverts lead with an introverted function.
Judging is a rational (based on fixed principles, active, and mainly conscious) process concerned with evaluating and sorting out informations, making decisions, forming opinions etc. If the judgement is introverted, its validity is given by internal and subjective criteria. If it is extroverted, it evaluates reality according to externally validated criteria.
Judgement is made through feeling (Fi, Fe) or thinking (Ti, Te).
Each type has two judging functions, feeling and thinking, one introverted and the other extroverted (Fe+Ti/Ti+Fe or Te+Fi/Fi+Te).
Perceiving is an irrational (independent from judgement, based on "impressions") process determining the way of absorbing informations from the inner or outer world. Perception can be either intuition (Ni, Ne) or sensing (Si, Se). Introverted perception draws its energy from the unconscious world, while extroverted perception is projected outwards and it is concerned with the external environment.
Each type has two perceiving function, sensing and intuition, one introverted and the other extroverted (Ne+Si/Si+Ne, Ni+Se/Se+Ni).

Patterns

Each type has two extroverted (e) and two introverted (i) functions, two judging (J) and two perceiving (P) functions with different orientation: in the end, each stack is composed by all four styles, with fixed patterns.
So, an introverted perceiving function (Pi) is always supported by an extroverted judgement (Je), and viceversa; and introverted judgement (Ji) is supported by extroverted perception (Pe).
The first and the last letter of each type are determined by these two elements (for example, INTP is Ti-Ne/Ji-Pe and ENTJ is Te-Ni/Je-Pi).

• IxxJ: Pi - Je - Ji - Pe Introverted perceiving with extroverted judging
• IxxP: Ji - Pe - Pi - Je Introverted judging with extroverted perceiving
• ExxJ: Je - Pi - Pe - Ji Extroverted judging with introverted perceiving
• ExxP: Pe - Ji - Je - Pi Extroverted perceiving with introverted judging

• xxxJ: dominant or auxiliary extroverted judging -> Te for xxTJs, Fe for xxFJs
• xxxP: dominant or auxiliary extroverted perceiving -> Se for xxSPs, Ne for xxNPs.

As you can see, both IxxJs and ExxPs lead with a perceiving process, while IxxPs and ExxJs lead with judging.

Typing by functions