My husband brought a test he got from a co-worker, to use on my fictional characters. We did the test ourselves too, but we forgot the results and we lost the test form!
(I'm fairly sure I scored ENTP, though...)
After a while, I wanted to find the test again because I wanted to type some more fictional characters. Then we sought on the internet. I've learnt about it a bit through some sites and this forum. I made a test myself (with Excel) because I wanted:
1) to answer the questions not only with "yes" and "no" but with "certainly not", "rather not", "neutral", "rather yes" and "certainly yes".
2) to get not only the type as result, but also the extremeness.
3) to represent the result in graphs (am I a nerd or what?
)
4) to save the results from those fictional characters.
I've never done the official MBTI test and I won't do it. I think I understand it well enough for what I want to do with it - making sure my fictional characters have different personalities.
As for functions, I've learned about them mainly through this forum. I found it very confusing at first. But by imagining the functions as little people living in my head, I constructed my understanding of them. By drawing the function guys and reading the reactions on them, I adjusted my understanding until it's more or less right.