Meem with Vowel Signs (مَ مُ مِ): This م is read with its vowel sign as normal. As for the م with sukun (مْ), it is written in the Mushaf in one of the following ways, based on its rules.
1. Clear Enunciation with the Lips الإظهار الشفوي - This م is represented in the Mushaf with a small top part of the letter ح on it. This rule applies when the م with sukun is followed by all the letters of the Arabic alphabets except ب and م. This rule requires that you pronounce the م clearly by closing the lips together.
Examples are: أيكم أحسن - عليهم صلوات
2. Hiding with the Lips الإخفاء الشفوي - This م is represented in the Mushaf without any vowel sign or anything at all on top of it. This rule applies when there comes after the م with sukun the letter ب. This rule of hiding the م applies with retaining the nasal sound (ghunnah) of the م, holding it for two counts.
Examples are: ءامنتم بالله - ونبلوكم بالشر
3. Merging الإدغام - This م is represented in the Mushaf without any vowel sign or anything at all on top of it. This rule applies when there comes after the م with sukun another م with a vowel sign on it. The second م will be written with a Shaddah(ﹽ) on it. The ruling is for both م to be merged while retaining the ghunnah for two counts.
Examples are: كنتم مؤمنين - ولكم ما كسبتم
**When you pronounce م with the Hiding rule, because it shares the same point of articulation with ب, you put the lips together without closing them, leaving a slight gap between the lips to produce the hiding sound.