Create a portal, add pages and modules and roles and permissions as you like, and then export the portal with content via the host, portals page.
The exported portal will be available on the template dropdown when a new portal is created.
However, there are caveats with exporting content and settings. You'll need to test it and see what works. There are also some bugs in the current DNN that cause portal skins to not be exported fully. Fixes are coming in Cambrian.
Rob