Note: You have to create at least one template before you can assign one using this method.
Start by accessing your lesson plan editor (video link)
From here you'll want to scroll to the bottom of the editor where you see the following buttons:
Select 'Choose Template' to get the following window
Using the drop down menu select what template you'd like to assign. This will replace the current contents of the lesson. This is irreversible so make sure you don't need any of that content before you confirm.