You'll need to know how to evolve Rufflet into Hisuian Braviary in Pokemon Legends: Arceus, if you want to complete your Pokedex. After all, that's the game's overall goal, and you won't be able to do that unless you unlock all the new evolutions and forms available in Hisui.
Hisuian Braviary is a little bit of an unusual one, as it is one of the special Pokemon you'll be able to summon later on in the game to help you better traverse the world. However, you can also have a Hisuian Braviary as part of your party by evolving one from a Rufflet. It's a great Pokemon because of its dual Psychic / Flying Type that's quite the formidable combo, so here's how to get your own.
1. Find a Rufflet
If you want your own Braviary, you'll need to get yourself a Rufflet. You'll find Rufflet gathering in the Lake Acuity and Heart's Crag areas of the Alabaster Icelands.
2. Raise your Rufflet to level 54
Then, rather than having to use any kind of evolution item, all that's required to evolve your Rufflet into Hisuian Braviary is raising it to level 54. Then you'll be prompted that your feathered friend is ready to evolve, and all you'll need to do then is select it in your party list and hit X to start the evolution process.
Alternatively, you can catch one in the wild
It's worth noting that Hisuian Braviary does appear as a rare spawn at Lake Acuity in the Alabaster Icelands. Evolving your own is definitely the easier option, but if you're lucky you may be able to find Braviary soaring through the sky around Lake Acuity. You'll need some Feather Balls (or better), and potentially to enlist the use of a Bean Cake - a lure that's perfect for flying Pokemon and will draw them down. The recipe for the Bean Cake can be purchased at the Craftworks in Jubilife Village.