Witr prayer, according to hanafi mazhab, is wajib. Which means that if you miss any, you have to do Qaza for it.
It is prayed in three rikaat; sitting between the second and third rikaat, then getting up, reciting fatiha and a short surah, then you say takbeer and raise your hands, then fold them back and recite qunut. After, qunut you say takbeer and do ruku, finishing the rikaat and sitting for tashahhud and salam.