To create new custom editable template you will need to define a template type. Template types effectively provide templates for a template. When creating a new template the structure and initial content of the selected template type is used to create to the new template.
Template types allow you to define:
The resource type of the page component.
The policy of the root node, which defines the components allowed in the template editor.
It is recommended to define the breakpoints for the responsive grid and setup of the mobile emulator at on the template type. This is optional, because the configuration could also be defined on the individual template.
Template types are typically defined by developers.
Once the template type is defined you can navigate to Tools -> General -> Templates -> Project Specific Folder -> Create the template.