This one should be pretty easy. Basically, you take a type and flip all the letters around. So if we were to take a say...Istp and flip that around we would get an Enfj. Now, there are some other aspects to this as well. You might have guessed that the cognitive functions would also be different as well. If you did you would be mostly right. Let's continue with the Istp example.
These are the cognitive functions of an Istp:
Driver: (Ti) Introverted Thinking (Accuracy)
Co-Pilot: (Se) Extroverted Sensing (Sensation)
10yr old: (Ni) Introverted Intuition (Perspectives)
3yr old: (Fe) Extroverted Feeling (Harmony)
And these are the cognitive functions of an Enfj:
Driver: (Fe) Extroverted Feeling (Harmony)
Co-Pilot: (Ni) Introverted Intuition (Perspectives)
10yr old: (Se) Extroverted Sensing (Sensation)
3yr old: (Ti) Introverted Thinking (Accuracy)
As you might have noticed the two inferior funtions of the Istp are the two dominant functions of the Enfj and the two inferior of the Enfj are the two dominant of the Istp. Because of this relationships with your opposite type can be hard but also benifitial. Because your strengths are their weaknesses and your weaknesses their strengths it can help make sure that you maintain a healthy relationship with your ten-year-old and three-year-old but it can also be kind of hard since you are well...opposites. Another problem with a relationship with the opposite type is that there is no real variety in the cognitive functions, you have the same four of the eight.
Enfp = Istj
Entp = Isfj
Esfp = Intj
Estp = Infj
Enfj = Istp
Entj = Isfp
Esfj = Intp
Estj = Infp