When creating an assignment, there is a checkbox to decide whether you want to enable guided reading mode. You can enable it for some or all students taking the assignment.
After you create the assignment, you will see it on your Assignments page. You will see a Guided Reading label in the corner of the image for the assignment if you have assigned guided reading mode to at least one student in the assignment. You can see this same label on the image on the assignment dashboard. You will also see a guided reading data section on the assignment report.
Finally, you can see if Guided Reading Mode is enabled, or enable/disable it, from your Edit Assignment wizard. Click the pencil icon in the top-right of the assignment to edit it.