Answer

I explain it there. The ruling is made in light of the tradition we received, but only a tradition that comes from Sinai, not inventions of previous generations. The reasoning of our predecessors is not binding, and each person must know it and formulate his own position on the matter he is dealing with. Exceptions to this rule are binding sources, such as rulings of the Sanhedrin, a law given to Moses at Sinai, and the Talmud. That is first-order ruling. Second-order ruling relies on non-binding sources only by virtue of the fact that the ruling appears in them (and not because I agree with what is written there).
I gave there the example of the mourner’s Kaddish, where there is no question at all on the halakhic plane, and that does not stop people from discussing it and bringing sources this way and that. Keep reading. The matter is explained there very clearly.
As for the question of how you know you ruled correctly—you don’t. A judge has only what his eyes can see. Even in second-order ruling you do not know, since there too there is interpretation and reasoning of the earlier decisor and of your own. Whoever seeks certainty will not find it. You need to rule according to what seems correct to you. When you reach the heavenly court, they will tell you whether you ruled correctly or not. But even if you did not rule correctly, don’t worry. As I explained there, in my opinion the Holy One, blessed be He, expects autonomy from us no less than He expects correct ruling.