The Sunnah method of shaving the head or shortening the hair is to begin with the right side of the head and then shave the left side. It is mentioned in the hadith:
“قَالَ لِلْحَلاَّقِ خُذْ وَأَشَارَ إِلَى جَانِبِهِ الأَيْمَنِ ثُمَّ الأَيْسَرِ ثُمَّ جَعَلَ يُعْطِيهِ النَّاسَ”
‘The Messenger of Allah, may Allah bless him and grant him peace, said to the barber, “Take it.” He gestured with his hand toward the right side of his head, then toward the left. Thereafter, he began distributing it among the people.’[1]
[1]. Muslim: 2298.