بسم الله الرحمن الرحيم
If the hair of head is less than one finger tip then it is not correct to do qasr (shorten) rather it is necessary to do halaq and halaq shall be done of complete head. And if there are long hair and they are more than one finger tip then qasr is right in this case and it is not necessary to shorten each and every hair of the head, rather qasr of one fourth hair is also enough.
ثم يحلق أو يقصر و الحلق أفضل ويكفى فيه ربع الرأس (مراقى الفلاح)
Allah knows Best!
Darul Uloom Deoband