Open the sheet that you want to mark as template.
Select the "Publish as template" option. You will see the option to select the category/ sub category/ group that you want to add the template to.
First select the category. You can select from the existing option in the drop-down, or you can create new sections by clicking on the + symbol next to "Category".
Similarly specify the sub-category and group as well.
Then, add the name of the template and click on "Submit".
A new template is now created, and whenever you select "Add sheet", this template will be available in the "Add from catalogue" option.