Navamsa means 9th part, which means each house of Rasi chart is divided in to 9 equal parts, 306 degrees divided by 12 gets to 12 zodiac signs of 30 degrees and each house when divided by 9 times gets to 3.33 degrees of each amsa, and then 9 such amsas of each house are spread across 12 houses, and for each lagna there is a different starting point, basically it means that we are zooming into each house and spreading it to see things in detail and when each house is disected into such fractions (amsa) when they are all placed over each other, like Rasi, followed by D2, D3, up to D60 and then see how many points are matching in each divisional chart, e.g. Ascendant in Rasi is Gemini and Ascendant in D2 is also Gemini, and D3 also, then it strengthens the Ascendant three folds, which is also called Vargottam meaning Fraction of high quality, similarly see where the Jupiter is in Rasi, and other charts if it matches its position particularly in Rasi and Navamsa, Jupiters strength increases by two folds being Vargottam.
Total there will be 108 amsas which are divided in to four groups,
1) Aries, Leo and Saggitarius for which amsas are spread from Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Saggitarius.
2) Taurus Virgo and Capricorn for which amsas are spread from Capricorn, Aquarius, Pisces, Aries, Taurus, Gemini, Cancer, Leo and Virgo.
3) Gemini, Libra and Aquarius for which the amsas are spread from Libra, Scorpio, Saggitarius, Capricorn, Aquarius, Pisces, Aries, Taurus, and Gemini.
4) Cancer, Scorpio and Pisces, for which the amsas are spread from Cancer, Leo, Virgo, Libra, Scorpio, Saggitarius, Capricorn, Aquarius and Pisces.
For Gemini Lagna, Start the first amsa with Libra, Scorpio, Saggitarius, Capricorn, Aqaurius, Pisces, Aries, Taurus, and Gemini, then start over with second house and so on and so forth.
Navamsa Chart is also read in similar manner like Rasi chart, so it has Ascendant, and remaining 11 houses similar to Rasi chart.
9th part is dharma amsa, so it relates to your dharma and also to your spouse which is your prathama dharma, followed by children, job, earnings, wealth, duties, social obligation, family obligation, etc. and it represents oversll fortune of the native.
Learning astrology start with south Indian chart which is very simple compared to North Indian and Western astrology (Iraq Astrology), the south indian chart is a square with four equal squares in each direction, the zodiac signs are fixed, the number of house changes with Ascendant, If Ascendant is Gemini then Gemini becomes the first house (Tanu bhava) and same for Navamsa and other divisional charts also.
There is RASI and Navamsa charts of Late Shri M K Gandhi (Father of Indian Nation) at http://ianala.blogspot.ca/search/label/Astrology
Ascendant rising (Lagna) or planet in 1st Navamsa in a chara rasi (movable sign), 5th Navamsa in a Sthira rasi (fixed sign), and 9th Navamsa in a Dwiswbhava rasi (dual sign) will be Vargottam hence gains in strength.
Lagna rasi or lagna lord should not form or occupy 6th, 8th or 12th house in Navamsa chart, and lord of 6th, 8th or 12th house should have any association with the Lagna rasi or Lagna lord.
Lords of 1st, 4th and 7th Navamsa are deva guna or Godly beings,
Lords of 2nd, 5th and 8th Navamsa are Nara or Humans,
Lords of 3rd, 6th and 9th Navamsa are Rakshasa or Demons, these terms should be understood strictly in terms of materialism and accumulation of wealth, deva or godly beings will earn and keep what they need there is lot of charity and humans might go both ways and there might be some charity, but Demons will just accumulate and accumulate for self and no or very little charity.
Vargottam planets are superior and fortunate,
Rahu and Ketu do not rule any Navamsa division.
Planet in its own Navamsa is superior then in its exalted Navamsa,
Planet in its Exaltation sign in Rasi chart but in its Debilitation sign in Navamsa produces bad results early.
Planet in its Debilitation sign in Rasi chart but in its Exaltation sign in Navamsa produces auspicious results.
Planets occupying Navamsas ruled by Jupiter act like natural benefics,
Saturn in its own house (Aquarius or Capricorn) or exaltation sign (Libra) does not afflict natural benefics provided it does not occupy its debilitation sign (Aries) in Navamsa chart.
A planet whichis very strong in Rasi chart does not produce desirable good results during its maha Dasha or Antara dasha with respect to that bhava or its lord, if it is placed in 6th,8th or 12th house from the bhava or its lord in question in Navamsa chart.
A planet as a lord of the 2nd house is in conjunction with lord of the 10th house in 10th house and the lord of the lagna is involved in raja yoga that native can secure a very high political or administrative position in the maha dsha or antara dasha of the lord of the 2nd house but If lord of the 2nd is in 6th,8th or 12th house from lordo f the 10th in Navamsa chart the resutls will be contrary.
BPHS: Navāńś. The Navāńś calculation are for a Movable Rāśi from there itself, for a Fixed Rāśi from the 9th there of and for a Dual Rāśi from the 5th there of. They go by designations Deva (divine), Manushya (human) and Rakshasa (devilish) in a successive and repetitive order for a Movable Rāśi. (Manushya, Rakshasa and Deva are the order for a Fixed Rāśi, while Rakshasa, Manushya and Deva are a Dual Rāśi’s order)