presitah-being told to execute the sacrifice; adhvaryuna-by the rtvik priest; hota-the priest in charge of offering oblations; vyacarat-executed; tat-that (sacrifice); samahitah-with great attention; grhite havisi-upon taking the clarified butter for the first oblation; vaca-by chanting the mantra; vasat-karam-the mantra beginning with the word vasat; grnan-reciting; dvijah-the brahmana.