I assume Shanks became a Yonko just by being strong himself and having a strong crew.
Since Shanks is part of the original four yonkos along with Whitebeard, Kaido and Big Mom, he didn't have to beat any other yonko to get the title. The four of them all got the yonko title for being in a stalemate from fighting each other.
As for the practical reasons, i think that being able to have duels with mihawk and possibly having the strongest active pirate crew along with some territory and subordinate crews would be enough for Shanks to be recognized as a yonko.
Since Shanks is part of the original four yonkos along with Whitebeard, Kaido and Big Mom, he didn't have to beat any other yonko to get the title. The four of them all got the yonko title for being in a stalemate from fighting each other.
As for the practical reasons, i think that being able to have duels with mihawk and possibly having the strongest active pirate crew along with some territory and subordinate crews would be enough for Shanks to be recognized as a yonko.