a. Download the source archive from CTAN,
or tagged as $VER$ in the Git repository by
<http://git.sourceforge.jp/view?p=luatex-ja/luatexja.git
- ;a=snapshot;h=$VER$;sf=tgz>
+ ;a=snapshot;h=refs/tags/$VER$;sf=tgz>
b. Extract the archive and put src/ into your TEXMF tree.
An example location is TEXMF/tex/luatex/luatexja.