To send a custom notification using Flow in Salesforce, create a Custom Notification Type in Setup, then add the Send Custom Notification action to a flow with four inputs: the notification type’s Id (from Get Records), a title, a body and a text collection of recipient Ids. The notification appears in the recipients’ bell and, if enabled, on the Salesforce mobile app.
I built a record-triggered flow in a Salesforce Developer Edition org that alerts the case owner when a High priority case is created, tested it with a High and a Medium case, and hit a real flow error along the way. Below are the exact settings, the notification that arrived, and the fix for that error.