<pre>
git clone git://git.xonotic.org/xonotic/xonotic.git
cd xonotic
-sh all-update.sh
+./all update
</pre>
After that, you have a working checkout of the repository.
<pre>
git clone ssh://xonotic@git.xonotic.org/xonotic.git
cd xonotic
-sh all-update.sh
+./all update
</pre>
If you already have a checkout, you would do:
<pre>
git config remote.origin.url ssh://xonotic@git.xonotic.org/xonotic.git
-sh all-update.sh
+./all update
</pre>
After that, you can write to the reposiotry using the usual git commands (commit, push).