stsci.sphinxext contains extensions to Sphinx to build documentation from STScI.
These instructions only cover changes from the standard Sphinx instructions.
Create a new tree for your Sphinx documentation, using sphinx_quickstart, as described in the Sphinx introduction.
Import the stsci_sphinx customizations into your project's conf.py by inserting the following line at the top:
from stsci.sphinxext.conf import *
The "extensions" line in your conf.py should also be modified so it doesn't override the extensions defined by stsci.sphinxext. For example, change:
extensions = ['sphinx.ext.autodoc']
extensions += ['sphinx.ext.autodoc']
Also, comment out the 'html_theme' line, so your project will use the stsci.sphinxext one.