Applying a Drupal Patch via Mac Terminal

1) Download the patch to the directory that the file being patched is in.

2) In terminal,
cd ~/Users/admin/my-website/modules/some-module-directory

Once you're in the correct folder, run this command using the exact name of the patch file: patch < my-filename.patch

If the patching was successful then you will see one or more lines such as "patching file my-file.module".

3) Clear your drupal cache

Source: Drupal.org