Well, when someone customizes a character, there are values that change like height, weight, hair cut, hair color, skin color, …
When a player is done customizing their character, save all those values. I would assume your character maker has some sort of struct or data set to store all those values, so you can probably just copy that data variable and store that copy in your GameInstance.
When a playable level loads, in your Player blueprint / class get a reference to the GameInstance, retrieve that data and apply those values again to the character.
I can’t give more details since I don’t know how your current system works, but as a general idea that would be the base logic.