This is a reminder for myself. Sometimes, rarely I discover bugs while running free software, if I can fix the bug it would be a pity not to attach a patch to the bugtracker to make the work of developers easier.
Creating a patch file:
diff -u old.file new.file > new.patch
Apply a patch using the patch command:
patch -p0 < patchfile
That is all one need to know to create and apply a patch on top of working directory.