There are fthree rules of Meem sakinah
1- izhar and it means to pronounce the meem clearly without doing anything and we apply it when the meem appers before all Arabic letters except meem and ba’.
2- idgham: to merge the meem sakinah sound with the following letter when it appears before meem
3- Ikhfa: hiding or concealing the meem sakinah when there is ba’after