The basic concept is simple: writing a text file and insert placeholders for properties of an object via braces {{property}}, which will be replaced by the actual data of the current object.
For example, in the pre-defined templates you will find:
/// {{Name}}
/// {{TypeName}}
/// OrderNumber: {{OrderNumber}}
/// FwVersion: {{FirmwareVersion}}
/// Author: {{Author}}
/// InstallationDate: {{InstallationDate}}
These placeholders will be replaced by the actual data of the current object, in this example by the properties of the current PLC during import.