The Shaykh (Al-Mufid), may Allah سبحانه وتعالى support him, said: "If she wants to purify herself through ghusl, she should examine herself with a piece of cotton. If blood appears on it, she is still menstruating and should delay the ghusl until she becomes clean. If the cotton comes out clean from blood, she should wash her private parts, then perform ablution (wudu) for prayer. She should begin with rinsing her mouth and nose, then wash her face, hands, and wipe her head and the tops of her feet. Afterward, she should perform ghusl by starting with washing her head, followed by her right side, and then her left side. If she omits rinsing her mouth and nose during her ablution, it does not invalidate her purification."
Commentary #10