If I want to make a patch for a debian package:
cd /tmp/; apt-get source package; cp -r package-1.1/ package-1.1.orig/
and then fix file(s) in package-1.1/ directory
/tmp$ diff -ur package-1.1.orig package-1.1/ | mail firstname.lastname@example.org
You should submit the patch via the BTS using reportbug esp. when the fix addresses an already reported bug and/or non-trivial like mine usually aren’t. And then applying the sodding thing:
cd suitable-directory; patch --dry-run --silent -pX < patchfile # X is a suitable number to remove elements from the pathnames in the patch file.
sam$ patch -p0 < /home/hendry/Mail/mail-attachments/maradns_1.2.03.3-1.1.diff