Importing an InDesign template is possible by clicking on ‘Maintenance’ on the top row and afterwards on ‘Template Builder’ on the left. In a new screen, the template builder will open.
Click on ‘Templates’ and in the dropdown menu ‘List templates’.
Afterwards, select ‘Import IDML’ or ‘Import Package’ depending on the file you wish to upload. If ‘Import IDML’ has been selected select your file and it will be uploaded.
When you have selected ‘Import Package’ select all the fonts, links and the INDD file. If there are for example no fonts or links please leave this field blank. Click on the ‘Import’ button to import the template to the template list.
This article also has a Youtube video to show how this action can be done.