Navigate to Settings > Preferences > Question Fields tab. Click 'Add New Field' button.
1) Select for Appointments or Classes (if you need it for both you can create two fields)
2) Link this policy to a specific service or all services
3) Enter your policy details
4) Select if it's optional or not
5) Select type: Checkbox
6) Click Save
When customers book online they will be required to accept your policy.