I've found that a lot of people train turnout by stretching. While this is important, you need to have the strength to hold the turnout. Clamshells are your friend. Also, turnout boards: they're really popular in the ballet world, and I have found that they've improved my turnout astronomically (https://www.amazon.com/Ballet-Turnout-Training-Boards-Discs/dp/B074C1L4PJ). You can do turnout exercises on the boards which really optimizes the benefit.
There's a ballet dancer named Kathryn Morgan that has a lot of technique videos on youtube that are helpful, specifically a few on turnout.