As we are going to be inheriting everything from the parent template this can be kept to the absolute bare minimum. I have created an example blank child template that you can download and customise.
Take my example file and replace the BTXXXXX with the name of your child template and update other values as required. You will see in my example that I add my initials to the name of the child template to avoid any possible conflict with other child templates.
Copy the files you created into their respective folders. The language files are optional but they should be edited and renamed as well and the same with the two preview images.
My example includes an empty index.html file in each folder. This is required because Joomla will not create an empty folder during installation and while you might not need those folders now you will thank me later.
Now just create a zip file of everything and test that there are no mistakes and that it will install on your test site.
All done. Now you can safely install on any new site
Wonder why you never thought of this before and rush to create installable child template of all your favourite overrides and share them with the world.
As well as the example blank child template I am also sharing some of my own installable child templates. They can serve as an example for you as they each use different features and functionality.