Basically you do this:
zcat ../foo-1.2-1.diff.gz | patch -p1 --dry-run
If patch does not complain, remove
This solution comes from jelly of irc.freenode.net#debian after being asked for instructions on how to apply a patch without having to understand ones action.
The alternative was reading one of many tutorials, for example the Arch Linux Build System one. I tried understanding patches and diff files in the past but my only result was aching head and having to ask for guidance in the end.