Source: here.
When the altar is piled, it is termed ugra. It is filled with the fury of rudra mahAdeva. The agni in the midst becomes rudra and the sacrificer readies to pacify him. He recites thus:
rudró vA eShá yád agnís tásya tisráH sharavyAH pratIcI tiráshcy anUcI
This fire is rudra, three are his missiles, one that strikes straight, one transversely, and one that strikes from below (These are the 3 stars of Orion, the invakas, which are the 3 arrows fired by rudra slaying prajapati).
tAbhyo vA eShá A vR^ishcyate yò agníM cinuté
By them he who piles the fire [altar] is cut up.
agníM citvA tisR^idhanvám áyAcitam brAhmaNAya dadyAt tAbhya evá námas karoti átho tAbhya evAtmAnaM níSh krINIte
Having piled the fire altar [the yajamAna] should give unasked a bow and three arrows to a priest; Indeed to them [the bow and arrows of rudra] he pays homage. Indeed he buys protection for himself from them. [The priest stands symbolizing rudra for the yajamAna as he is given the bow and the 3 arrows and with that gift he is buying protection from rudra]
yát te rudra puráH dhánus tád vAto ánu vAtu te tásmai te rudra saMvatsaréNa námas karomi
O rudra , in the east [is] your bow, may the wind blow after it for you, to you, O rudra , with the first year [of the five year calendrical cycle] I pay homage.
yát te rudra dakShiNA dhánus tád vAto ánu vAtu te tásmai te rudra parivatsaréNa námas karomi
O rudra , in the South [is] your bow, may the wind blow after it for you, to you, O rudra , with the second year [of the five year calendrical cycle] I pay homage.
yát te rudra pashcAd dhánus tád vAto ánu vAtu te tásmai te rudredAvatsaréNa námas karomi
O rudra , in the West [is] your bow, may the wind blow after it for you, to you, O rudra , with the third year [of the five year calendrical cycle] I pay homage.
yát te rudrottarAd dhánus tád vAto ánu vAtu te tásmai te rudreduvatsaréNa námas karomi
O rudra , in the North [is] your bow, may the wind blow after it for you, to you, O rudra , with the fourth year [of the five year calendrical cycle] I pay homage.
yát te rudropári dhánus tád vAto ánu vAtu te tásmai te rudra vatsaréNa námas karomi
O rudra , in the Zenith [is] your bow , may the wind blow after it for you, to you, O rudra , with the fifth year [of the five year calendrical cycle] I pay homage.
rudró vA eShá yád agníH sá yáthA vyAghráH kruddhás tíShThaty eváM vA eShá etárhi sáMcitam etáir úpa tiShThate namaskAráir eváinaGM shamayati
rudra is verily [manifest] in this fire; just as tiger stands in fury, so also he stands. When the [fire-altar] is piled with these [bricks], he worships [rudra], and indeed with homage he soothes him.
yo rudro agnau yo apso ya oShadhIShu yo rudro vishva bhuvanA .avivesha tasmai rudrAya namo astu ||
rudra in the fire, in the waters, in the plants, rudra who has entered all entities of the universe, to that rudra homage.
He makes pourings of soma and fire offerings of the gAvIdhuka charu to the terrible rudra with the shatarudriya incantations. Finally with the above formula he makes an oblation of gAvIdhuka charu on the last brick of the altar to soothe and pacify rudra.
For the kApAlika observing the mahAvrata, these five fold mantras are the ones with, which he worships the AmnAyas of rudra. The body is his altar. His own skull is his soma cup. His food is the gAvIdhuka charu. Having fed the heads of rudra with the internal soma, streaming from the chandra nerve he realizes the amnAya know as anuttara. He then recites: “rudró vA eShá yád agníH sá yáthA vyAghráH kruddhás tíShThaty eváM vA eShá etárhi sáMcitam etáir úpa tiShThate namaskAráir eváinaGM shamayati“ The face of the anuttara manifests, like the crocodile face of tumburu.