Tantrasara (Krishnananda) 2.p25

2.p25

ācamya vidhivanmatrī śucau deśe ca saṃviśet .
jale saṃyojya tīrthāni trivāraṃ mūlamantrataḥ .
kṣipedbhūmau kuśāgreṇa saptadhā mūrdhni secayet ..

Having sipped water according to rule, the mantrin should sit in a clean place. / Having brought the sacred waters together in the water, he should cast it three times / onto the ground with the tip of a kuśa grass with the root-mantra, and sprinkle it seven times onto his head.