How it is worked out
Count the nakshatras from the one the Sun is in to the one the Moon is in, including both. Divide the count into groups of three: nakshatras 1 to 3 belong to the first graha, 4 to 6 to the second, and so on through nine groups. The nine grahas, in order, are: Surya, Budha, Shukra, Shani, Chandra, Mangala, Guru, Rahu and Ketu.
For example, if the Moon is in the eighth nakshatra counted from the Sun's, it falls in the third group, Shukra's, a good day for a homa. If it is in the eleventh, it falls in the fourth group, Shani's, which is avoided. The result changes as the Moon moves into a new nakshatra, about once a day.
When it matters
Homahuti is checked, with Agnivasa, when choosing a day for an optional homa: a havan for health, wealth or a wish, a graha shanti or a homa at the start of a venture. Days on which the ahuti falls in the mouth of Budha, Shukra, Chandra or Guru are preferred.
As with Agnivasa, it is not checked for daily fire offerings, for homas that belong to a samskara, or for homas on fixed festival days.