How do we correctly patch our core and modules in drupal?

First, run

Composer require cweagans/composer-patches

Now you have a package installed that enables you to patch core, libraries or modules.

Second, add a patch to a folder named 'patches' in the root (where your composer.json file is).

Third, add an "extras" section in composer with a patch:

"extra": {
    "patches": {
        "drupal/core": {
            "Undocumented title variable in feed-icon.html.twig": "patches/3156260-11.patch"

Fourth, run

composer install

This will remove the core, re-download and patch it. Same goes for modules and libraries.

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