How do I apply a patch supplied in the issue queue via Git?
Applying a patch to a standard install doesn't require you to switch it over to Git. You can just apply the patch from a given PR (let's say it is PR #12345) like so from the command line:

cd path/to/civicrm
wget https://github.com/civicrm/civicrm-core/pull/12345.diff
patch -p1 &lt; 12345.diff
rm 12345.diff