Finding Karlach in Baldur’s Gate 3 is relatively easy. She can be found in Act 1 after you have completed the prologue quest “A Nautiloid in Hell”. Once you have made your way to the Emerald Grove, slightly northwest of where you crash land on the beach, you’ll have easy access to her location. Once you rescue the grove from the goblin incursion, you’ll notice Wyll, The Blade of Frontiers, joins the fight. He will end up at the Emerald Grove training young Tieflings for the possible fight ahead.
When you chat with Wyll, make sure to ask him to join your group and the conversation will automatically lead to Karlach. The devil he mentions is there to hunt.
You will then have the general location of Karlach. The easiest way to find her and recruit her for your party is to leave Wyll at camp. That way there will be no bad blood when you first meet.
To be safe, avoid the first objective “The Blade of Frontiers” and continue east down the trail, past the dying Hyenas. They can be avoided by sneaking by and not interacting with any of the downed animals. Otherwise, a long and possibly difficult fight will ensue.
Karlach is just across the river. Chat with her and respond positively and offer to help her fight the Paladins of Tyr. There are no negative consequences for dealing with them, as they are pretending to be Paladins and are enemies in disguise.
Once you help Karlach with the Paladins, she will happily join your party. She will either stay with your group, or you can send her back to camp. The story between her and Wyll will continue at a later point in the game.