Instructions
Rest your most powerful or highest level Pokemon so they are at full health.
Travel to Feldera Volcano, Dolce Island, Tilikule Monument, Sobiana Road, and the Undersea Cavern. At the end of each dungeon in each location is a giant beacon. Walk up to the beacon and press the "B" button to activate it.
Walk outside of any cave or dungeon after activating all five beacons. Latias should attack you within a few minutes.
Use your best Pokemon to bring Latias down to under 25 percent health, then use a Master Pokeball to catch it.
Use the in-game menu for Latias and scroll down to "Abilities." Select the "Call Ranger Sign" action to call Latias to you. Heal Latias so it is at full health before you ride it.