The new version of Farfetch'd must meet a very specific set of criteria in order to evolve in Pokemon Sword and Shield. One of the new Pokemon featured in Pokemon Sword and Shield is Sirfetch'd, a new evolution for Farfetch'd specific to the Galar region. While Farfetch'd are usually happy-go-lucky birds, the Farfetch'd in the Galar region are hardened warriors that carry around massive leeks to use in battle. These Farfetch'd are exclusive to Pokemon Sword and players should be able to add one of these Pokemon to their teams relatively early during their journeys.
While Farfetch'd does evolve into Sirfetch'd, players have to complete a very specific requirement that isn't linked to leveling up or holding any items. Instead, Farfetch'd must score three critical hits during a single Pokemon battle in order to trigger its evolution. Of course, this is made easier when Farfetch'd holds a Leek, an item that increases Farfetch'd's critical hit rate, or by using attacks like Leaf Blade, which have boosted critical hit rates.
There are no other current requirements to evolve Sirfetch'd - once you score the critical hits in a single battle, Farfetch'd will evolve regardless of its level. This is a tricky evolution, and a key might be to bring a Farfetch'd into a battle where there are a lot of high level opponents that can soak up damage.