Rule 1: Doubled nūn and mīm: the fullest ghunna
Whenever ن or م carries a shadda, the ghunna must be held for two counts. This is the strongest and most complete ghunna, and it applies wherever the doubled letter appears, in the middle of a word or at its end, in continuation or at a stop. Say the first half of the doubled letter with the nasal hum, hold it, then release into the vowel: إِنَّ is in-na with a held nasal n.
إِنَّا أَعْطَيْنَاكَ
innā aʿṭaynāka
indeed We have granted you
doubled nūn with full ghunna (108:1).
اَلنَّاسِ
an-nāsi
mankind
the nūn is doubled after the sun-letter lām; hold the ghunna (114:1).
حَمَّالَةَ الْحَطَبِ
ḥammālata l-ḥaṭabi
the carrier of firewood
doubled mīm with full ghunna (111:4).
عَمَّ يَتَسَاءَلُونَ
ʿamma yatasāʾalūn
about what are they asking one another
doubled mīm (78:1).
ثُمَّ كَلَّا
thumma kallā
then no!
doubled mīm; the doubled lām in كَلَّا has no ghunna (102:4).
مِنَ الْجِنَّةِ وَالنَّاسِ
mina l-jinnati wan-nās
from jinn and men
two doubled nūns in one phrase (114:6).