Steps not shown are: Go to your Workflows, Click Create New Workflow, Select the Project you'd like to apply this workflow to.

Now select your Rule trigger/condition, for example, "If task is associated with tag..."

Select the appropriate tag from your list.

Select "Then send an email notification to"

Select who you want to send the email notification to. In this example, "an email address in a custom field"

Select the appropriate custom field from the list.

flowsana.net

This is an example of a Rule workflow with the action "send a custom email notification". It demonstrates how you can customize the email notification with Variable Substitution, as well.