From Pandoc - Pandoc User’s Guide,
--data-dir=DIRECTORY
: Specify the user data directory to search for pandoc data files.
If this option is not specified, the default user data directory
will be used. On *nix and macOS systems this will be the pandoc
subdirectory of the XDG data directory (by default,
$HOME/.local/share, overridable by setting the XDG_DATA_HOME
environment variable). If that directory does not exist and
$HOME/.pandoc exists, it will be used (for backwards compatibility).
On Windows the default user data directory is
C:\Users\USERNAME\AppData\Roaming\pandoc.
You can find the default user data directory on your system by
looking at the output of pandoc --version.
Data files placed in this directory (for example, reference.odt,
reference.docx, epub.css, templates) will override
pandoc's normal defaults.
mkdir -p "$HOME/.local/share"
cd "$HOME/.local/share"
git clone git@github.com:ickc/pandoc-datadir.git pandoc
cd pandoc
make installapa-6th-edition-custom.csl is modified from apa-6th-edition.csl to change the sort orders, etc. See make diff for the differences.
This has to be manually updated if apa-6th-edition.csl is updated upstream.