To practice problem structuring in a case interview I recommend practicing  the process of setting a hypothesis and devising an issue tree that tests your hypothesis.

It is okay if you are slow at the process in the beginning… the key is that you understand the process. For better proficiency and speed, you just need to practice.

You can force yourself to use the process every day for everyday decisions. Should I eat in tonight or go out to dinner? What’s your hypothesis and how would you test it?

You don’t actually need to go through the data gathering steps, but instead just practice structuring the problem or “setting up” the structure.

Try to do it three or four times a day.

Should you take the highway or the local roads to your destination?

Should you eat vanilla ice cream or chocolate?

Should you bring an umbrella or leave it at home?

Have your friends or family ask you these kinds of questions and force you to set it up.

Do it until you’re completely sick of it, and if you were woken up in your sleep at 3am, you could form a hypothesis, structure it, and go back to sleep without even fully waking up to do it (not literally… but you get the idea).  This will help you to ace the case.