It has three rules: 1 - The oral Idgham: It is to insert the sakin /m / into another voweled / m /at the beginning of the succeeding word, so that they both become one stressed / m /. This is called a lipial Idgham or identical with observing the pronunciation with the full resonance. 2 - The lipial concealing: It is to hide the  initial / m / which is preceded by a / b / sound. 3 - And The lipial clarification: It is to clarify the / m / if it is followed by all the sounds except for the / m / and the / b /. It is the clearest if followed by the / w / or the / f /. Fourth: The rules of the stressed / m / and the / n / sounds in case of sequence or stoppage, whether they occur in the middle or at the end of a word. The length of the resonance is for two harakahs.