First, please make sure you've added a standard set to your subject.
Now follow these steps:
- Access the Lesson Plan Editor (video link) from your Planner
- At the bottom of the lesson click 'Add Standards' (shown below)
Click it and a window will pop up which lets you choose the standard codes you want to attach to the lesson. You can search by code or key word using the built-in search bar (see below)
From here you can also remove standards that you've already assigned.