Pokémon have always had various ways of evolving into new beasties, and Pokémon Sword and Shield are no different. One of these curious contenders is the majestic Galarian Farfetch'd. You can find them primarily on Route 5, just east of the town of Turffield, but only in Pokémon Sword (if you own Pokémon Shield, you'll have to trade it over from someone with a copy of Sword). You can find them milling about in the overworld, so as soon as you see one, just run into it and say hello by squeezing it into a tiny ball.
Once you've caught one, take a look and see if it's holding a Leek as a held item, which is quite common. If not, keep catching more until you get one that does, as it'll be seriously handy later on.
To evolve it, all you need to do it get it to perform three critical hits in a single battle. This can be aided by having them hold the Leek, as it'll boost the likelihood of said critical hits happening. If you're in the earlier parts of the game this shouldn't be too difficult, but finding a trainer with at least three Pokémon will make it much easier, as you'll have more fodder to hit with those charming criticals.
If you're in the late game, however, this is going be a lot trickier, as in all likelihood you don't have any similarly-levelled trainers around you - that haven't already been defeated - in order to be able to get all three critical hits that you need before defeating the enemy. In which case we suggest going to the Wild Area and finding a fishing spot. Fish there with a strong Pokémon at the head of your party (just in case your Farfetch'd can't successfully run away) until you fish up a Magikarp. In the late game these things can be Level 60, but as they're Magikarp they're still phenomenally weak.
Switch your Farfetch'd in and start hitting the Magikarp with any old move, as long as it inflicts damage. You may get some bother from the enemy using Flail and Tackle, but it's also just as likely to use Splash, which does no damage at all, giving you a free hit. Keep your Farfetch'd's health appropriately high and you should get all three critical hits in before the fight is over.
If you're having trouble keeping your Farfetch'd from getting knocked out (Flail, we're looking at you), you can always disable one of the Magikarp's moves as long as it isn't Splash, and even Paralyse it for added benefit.
Regardless of how you get the three critical hits, once the battle is over your Farfetch'd will evolve into the gallant Sirfetch'd! What a handsome devil!