Simply type out an email with the required information.
As shown on the right there are various parameters that can be included, for example $ALERTNAME$ will be subsitiuted with the name of the alert that caused this email to be triggered.
A list of variables can be found here. The alert specific variables are shown on the right and in the standard alert examples in MCS.
When the email is complete click Save Template.
This template can now be chosen from the drop down menu when creating an alert.