In the Admin Dashboard, click on the Lessons tab and then on the Activities sub-header. Click on blue "New activity" button in the top right corner to create a new activity. The Create Lesson Activity page will appear. Complete all required fields marked with a red asterisk (*) and any additional fields you'd like to include.
Click on the Settings tab to specify which classrooms this activity is intended for. Click "Save" in the top right corner.
HERE is how to edit an existing lesson activity.