You can create scriptable templates with a text/HTML editor outside of NetSuite, or you can use the Text Editor field on the Template subtab of the template record itself. If you create a scriptable template outside of NetSuite, save it as a plain text (.txt) or an HTML (.html or .htm) file.
You may reach out to your Account Manager if you wish to have a developer to set you up a system email template according to your system requirements.
When updating system email templates (at Setup > Company > Company Management > System Email Templates) you can update the standard scriptable version of a system email template, by editing the template appended with 2.0.
When sending email or fax messages from transaction or entity records, if you want to compose the message on the message (rather than choosing an existing template to use), select the type of template in the Template Format field. Then, you can use the Field Type list and the Insert Field list to add fields to your message.