There aren’t many powerfully useful documents introducing patches to an EmacsNewbie. Its more often a required trade of software package maintainers and developers. However, the skill can be as useful to every day users (“the emacsen laity”).

The patch command is well documented, however, but mostly for an audience of those who already understand and use patches.

And also DiffMode and Ediff are well documented for Emacs.

See also InstallingPackages

To interactively apply a patch file, bit by bit, using Diff Mode in Emacs:

Diff Mode can be used to selectively remove changes from a file. This can be if you have already applied a patch. It can also be with respect to revision control, when you want to selectively back out some recently committed changes. In that case a patch can be provided by the revision control tool, or this can be done from the Emacs *vc-diff* buffer (which is already in Diff Mode) if using VC mode in Emacs.