One of the best exercises I know of on how to learn and practice User Story slicing techniques is the so called Elephant Carpaccio exercise. At Spotify it is something of a staple as it it is (often) used when introducing new employees (now a days).
The exercise is about creating a quoting application which includes different markets, tax and discounts. If you have not done this before your initial slices will probably be pretty large. The aha moment is when you realize how SMALL you can actually make them. You can can dry run this exercise by only creating and discussing the backlog. It’s also very friendly to actually do it for real by programing the application; even excel can be used to do that.
Henrik Kniberg has written an excellent guide on how to facilitate this exercise. Here’s my slides based on that presentation to make it a little bit easier to remember and run it in a classroom.