Generally through a conversation between the player and GM. If you are the GM, I recommend helping the player work on crafting their Experiences so you can both understand the intent behind them. While not Daggerheart per se, here is some advice on creating good aspects for Fate. You can easily port some of the advice from this game to Daggerheart.
Yup, and the book tells players this!
“Before creating Experiences, you’ll want to have a solid idea of who your character is; the background questions are especially helpful for this. When you’re ready, work with your GM to create two Experiences that have shaped your character.”
If it’s not clear to both sides what the Experience means and some places where it might apply - time to talk it out.
I absolutely agree that they could’ve used some more guidance around the Experiences, for each of my frames I’ve added in additional bits to walk the players on to some character shaping to get to the tone I was looking for.