Shadda is formed by bringing together two of the same letter: the first one is with jazm, the second one with a vowel. It is recited with emphasis.
Shadda is written like a “w” or an upside down “m” above the letter.
For example: اِنْ نَ , in this word, two nuns have come right next to each other. The first is with jazm, the second with a short vowel, so they are written as one nun with a shadda above اِنَّ and recited with emphasis. Transliteration: the first one is ‘in-na,’ the second is ‘innnna’.
A letter with a shadda does not begin a word.
Kasra may be written under the letter or under the shadda.
When Nun and Mim have a shadda they are given more of a ghunna sound than other letters.