Navigate to Settings > Phone Numbers > Advanced Settings > Call Scripts
Only Owners, Admins and Account Admins can create Call Scripts. Non-admins can't create Call Scripts but they can use it when on a call.
Make a call using the web dialer
Click on Scripts
Select Script
This is available via Labs right now. Enable it within Settings > Labs > Call Scripts
This works on the web dialer currently. Mobile app support coming soon.
Currently, you can't link the Call Scripts to a workflow, user or opportunity.