How do I set up a workflow to notify someone if a predecessor task has been completed?


I am currently trying to develop a project plan sheet, and I am wanting to be able to notify someone when the predecessor task was completed so they know they can start working.

For example if John has to do a Data Source Identification and validation but he can't do it till Mark has converted and cleaned up the data I want to be able to automatically notify John when Mark marks his conversion and data cleanse row as completed. How would I go about doing this?