We know that each sign is 30 degrees and when divided by 9 Navamsha means 9 divisions , the unit part will be 3.33 degrees. Each of these units is a distinct navamsa and can be labeled from 1 through to 9.
Aries has 9 divisions Aries, Taurus, Gemini, Cancer , Leo , Virgo , Libra, Scorpio, Sagittarius ,
For taurus first navamsha is CAPRIORN,
For Gemini its Libra
and for Cancer its cancer itself.
The pattern will repeat for Leo - Aries first Navamsha
Virgo - Capricorn as First Navamsha
Libra as Libra first navamsha
and scorpio , first Navamsha is CANCER.
above calculation can be summerised as :
a. Aries, Leo, and Sagittarius — the first navamsa will be Aries, b. Tauras, Virgo and Capricorn — the first navamsa will be Capricorn, c. Gemini, Libra and Aquarius — the first navamsa will be Libra, d. Cancer, Scorpio, and Pisces — the first navamsa will be Cancer
Say, for example, Moon is 5 degrees in the sign of Pisces , then it will occupy the second navamsa of Cancer which is leo ( as 3.3 degrees is first navamsha , above 3.3 is second navamsha)
eg: If planet is 20 degrees 55 minutes in cancer , 20.55 / 3.3 is 6.22 that is 7th navamsha for cancer , which is capricorn.