Assigning a pathway gives a classroom access to it, and lets you set when it opens and when it is due.
Assigning it
Open My Library from the sidebar and click your pathway. Set the type filter to Pathways if you have a lot of content.
Click Assign in the top right.
Assigning takes three steps. First, tick the classrooms you want and click Next. Classrooms that already have the pathway are listed under Already Assigned.
Next, check the dates. Students can start straight away and the pathway is due the coming Friday at midnight, unless you change it. Last, check the summary and click Assign (it reads Publish and assign if any of its lessons still need publishing).
You can assign one pathway to as many classrooms as you like, each with its own dates. See Setting a Start and Due date for a Pathway.