I think the biggest part of properly understanding body language is being an extrovert/being social. I could be wrong. A shy, introverted INFP is not going to develop any social acumen by reading body language tips.
On a first date, lean in (way in) to the person directly across from them and stare intensely into your date's eyes. Don't blink. Never blink. Open your eyes as WIDE as possible so s/he knows you are awake and paying attention. Don't smile, laugh, or otherwise react to what s/he is saying. That is rude and takes the attention off of them. Just keep staring. HARD.
“If you want to tell people the truth, make them laugh, otherwise they'll kill you.” ― Oscar Wilde