I want to show the original/list price and sale price (actual price) in my store as well as the discount percentage: N% off!
In Drupal 7, there is the excellent Commerce Price Savings Formatter module-- just install the module, set the prices, and you're done.
What can I do in Drupal 8? I have tried to use the built-in promotion feature in Commerce 2, but as far as I can tell the promotion UI can only be used to create coupons, not discounts that automatically apply to the relevant orders. (based on a reading of the documentation and experimenting with the module).
Here's a basic way to implement this in Commerce 2 if you don't need everything done automatically.