The 20% Plus discount is applied directly to the courses and course bundles available on the web, you don't need to do anything! As it is additional, for the purchase of individual courses you can use another promotion or discount code in your order. In the case of the purchase of a course bundle, only the Plus discount will be added automatically, it will not be possible to add another discount code as the bundles already have a reduction included.
When you log in with your Plus account, you will see all prices with the 20% discount already applied but once you add the course(s) to your cart, you will see the prices are broken down and the application of this and any other promotional code:
Remember that the discount percentage is not added, but applied to the price of the course at that moment, for example, if the course has a 10% discount (due to a promotion or a discount code already applied), the additional 20% would be applied on the already discounted price.
If you want more information on how to use your Plus credits, you can check this article.
The Plus discount will not be applied to courses purchased as gifts or gift cards.