To add standards to a lesson plan, click "Search & Add Standards" in the standards box at the top of any lesson. The first time you click that button for a class, Cc will ask you to choose a state/organization, content area, and grade level. Check out this article first if this is your first time using standards.
Once you've added your standards set, you can search through your standards by typing in a keyword or phrase in the search box at the top of the modal. Click as many standards as you would like and then click "Close" at the top right to return to your lesson plans. Any standards you have selected will be added to your lesson plans automatically.