First you should go to Settings menu, Options, Email tab and introduce the information related to email server settings.
To customize the email template just edit in the Email template textfield. There are a few placeholders that can be used to position some data in relation with the remaining text.
The available placeholders are:
#Resource - resource name
#StartDate - beginning of the time period considered for the assignment data
#EndDate - end of the time period considered for the assignment data
#ResourceAssignmentsTable - table with information about assignments for selected period
With every email, an .ics file is generated as an attachment. This is a standard way to store calendar information in a text file. The format is used for example by iCal on Macintosh and Google Calendar. Assignments can be defined as VEvent-s or VToDo-s, depending on your necessities.