Diphthongs sounds are a combination of two vowel sounds that are pronounced within the same syllable.
The ɔɪ sound is a combination of /ɔ:/ and /ɪ/ or /i:/. Like vowels the diphthongs are all made through the mouth and are voiced which means that you vibrate you vocal chords to make the sound.
Unlike with vowels and consonants, when you pronounce a diphthong sound, your mouth will change position and shape as you say the sound, because they combine two vowel sounds.
So as you say the ɔɪ sound your mouth will start on the position of the /ɔ:/ sound and finish on the position of the /ɪ/ or /i:/ sound.