Go to discounts in the left side menu, and in the upright corner you will find "create discount" button

Name your discount code and what type of code; choose between single purchase and subscription depending on which typ of product you want the code to be applicable to

Choose what product/products you want your code to be applicable to

Apply a discount amount or percentage depending on which one you prefer

Last step is to set the discount duration and restrictions if you want to further control the useage of the code.

Create and then the code is ready to use