Apastambagrihyasutra 4.10: daśamaḥ khaṇḍaḥ

We shall explain the Upanayana (or initiation of the student).

Let him initiate a Brâhmana in the eighth year after the conception,

A Râganya in the eleventh, a Vaisya in the twelfth year after the conception.

Spring, summer, autumn: these are the (fit) seasons (for the Upanayana), corresponding to the order of the castes.

(The boy's father) serves food to Brâhmanas and causes them to pronounce auspicious wishes, and serves food to the boy. (The teacher?) pours together, with the first Yagus (of the next Anuvâka, warm and cold) water, pouring the warm water into the cold, and moistens (the boy's) head with the next (verse; M. II, 1, 2).

Having put three Darbha blades into his hair (towards each of the four directions) (the teacher [?]) shaves his hair with the next four (verses; M. II, 1, 3-6) with the different Mantras, towards the different (four) directions.

With the following (verse, M. II, 1, 7, somebody) addresses him while he is shaving.

Towards the south, his mother or a Brahmakârin strews barley-grains on a lump of bull's dung; with this (dung) she catches up the hair (that is cut off), and puts it down with the next (verse; M. II, 1, 8) at the root of an Udumbara tree or in a tuft of Darbha grass.

After (the boy) has bathed, and (the ceremonies) from the putting (of wood) on (the fire) down to the Âgyabhâga oblations (have been performed), he causes him to put a piece of Palâsa wood on the fire with the next (verse; M. II, 2, 1), and makes him tread with his right foot on a stone to the north of the fire, with (the verse), 'Tread' (M. II, 2, 2).

Having recited the next two (verses; M. II. 2, 3. 4) over a garment that has been spun and woven on one day, and has caused him, with the next three (verses; M. II. 2, 5-7), to put it on, he recites over him, after he has put it on, the next (verse; M. II, 2, 8).

He ties thrice around him, from left to right, a threefold-twisted girdle of Muñga grass with the next two (verses; M. II. 2, 9. 10), and (gives him) a skin as his outer garment with the next (verse; II, 2, 11).

To the north of the fire (the teacher) spreads out Darbha grass; on that he causes (the boy) to station himself with the next (verse; M. II. 3, 1), pours his joined hands full of water into (the boy's) joined hands, makes him sprinkle himself three times with the next (verse; M. II, 3, 2), takes hold of his right hand with the next (formulas; M. II, 3, 3-12), gives him with the next (formulas; M. II, 3, 13-23) in charge to the deities (mentioned in those Mantras), initiates him with the next Yagus (M. II, 3, 24), and murmurs into his right ear the (Mantra), 'Blessed with offspring' (II, 3, 25).