How to generate Templates with Apples Pages?
Are you asking how to turn an existing document into a template, or are you needing advise on how to make the document?

If it's the former, you just need to save your document as a template using quite simply File > Save as template.

If it's the latter, then judging by the link you mentioned, I would start with a blank document in page layout format.  Select View > Show Layout and Show Rulers.  Start with your page margins, and set them to that shown in your example.  Then add text boxes that you can size and align against the rulers to make sure that they are in the correct places for the addresses etc.  For the section under the address that is 5 columns, you could use a table, using the cell properties to ensure the middle column is the desired width.

I can't read German, so am unsure what a lot of the notes mean, but I hope this is a start for you.