There are multiple ways in which Raj Yoga is formed. One for the main Raj Yoga is as follows:
When a planet gets the lordship for both Kendra and Kona, it becomes RajYog Karak for that Kundali. For Example, for Libra Ascendant, Saturn owns the 4th and 5th houses. Hence, Saturn becomes Raj Yog Karak for Libra Ascendants. Such Raj Yog Karak planet, when positioned in Kendra or Kona, gives its best results.
For Taurus and Libra Ascendants, ‘Saturn’ is Raj Yog Karak.
For Capricorn and Aquarius Ascendants, ‘Venus’ is Raj Yog Karak.
For Cancer and Leo Lagnas, ‘Mars’ is Raj Yog Karak Planet.
For Aries and Scorpio Ascendants, ‘Sun’ and ‘Moon’ together are said for be Raj Yog Karak.
This kind of Raj Yog is not seen in Mithun, Kanya, Sagittarius and Pisces Ascendan