There are 32 Ominous Stakes in S/V that you need to pull, 8 for each Ruinous Legendary. You can find their locations at this link. Every two stakes pulled of the same color will make one of the chains on the shrine door break off, and when you've pulled all the stakes of a color, you'll hear the Legendary's cry. When you go to the shrine, you can battle them.
If you kill the Legendary it will respawn in one in-game day, and if you don't want to wait, you can change the date in the Nintendo Switch settings.
Orange stakes = Chien-Pao
Yellow stakes = Ting-Lu
Purple stakes = Wo-Chien
Blue stakes = Chi-Yu
Hope this helps!
