
Hi Dpiet,
For using DocumentsCorePack to create a letter template, you can refer following lonk:
Also, you can try to use document template to create a letter template.
Go Settings > Template > Document template.
Clicking new button to select letter entity to create a letter template.
For more detailed steps, you can refer following link:
https://docs.microsoft.com/en-us/dynamics365/marketing/manage-word-templates