Ending of Mundaka Upanishad talks about śirovrata (शिरोव्रत):
तदेतदृचाऽभ्युक्तम् । क्रियावन्तः श्रोत्रिया ब्रह्मनिष्ठाः स्वयं जुह्वत एकर्षिं श्रद्धयन्तः ।
तेषामेवैतां ब्रह्मविद्यां वदेत शिरोव्रतं विधिवद् यैस्तु चीर्णम् ॥ मुण्डकोपनिषत् ३-२-१०॥
III-ii-10: This (rule) has been revealed by the mantra (which runs thus): 'To them alone should one expound this knowledge of Brahman who are engaged in the practice of disciplines, versed in the Vedas, and indeed devoted to Brahman, who personally sacrifice to the fire called Ekarsi with faith, and by whom has been duly accomplished the vow of holding fire on the head.'
तदेतत् सत्यमृषिरङ्गिराः पुरोवाच नैतदचीर्णव्रतोऽधीते ।
नमः परमऋषिभ्यो नमः परमऋषिभ्यः ॥ ११॥
III-ii-11: The seer Angiras spoke of this Truth in the days of yore. One that has not fulfilled the vow does not read this. Salutation to the great seers. Salutation to the great seers.
From the translation, it seems that it's "a vow of holding fire named Ekarshi on head" or a fire ritual.
I want to know what is the ritual of holding a fire named "Ekarshi" on head, called "Shirovrata"? Do we have any details/description available on how this ritual is performed?