Drupal 8/CiviCRM has the composer-patches package already installed. So you can add a section to the "extra" element in composer.json called "patches", and list out the patch with a title and a URL (or path) to a .diff file. Here's an example where I add four patches to standard CiviCRM:
diff --git a/composer.json b/composer.json
index 453ff74..5827a69 100644
--- a/composer.json
+++ b/composer.json
@@ -68,6 +68,14 @@
]
},
"extra": {
+ "patches": {
+ "civicrm/civicrm-core": {
+ "core#609, PR 13390.": "https://patch-diff.githubusercontent.com/raw/civicrm/civicrm-core/pull/13390.diff",
+ "core#580, PR 13373.": "https://patch-diff.githubusercontent.com/raw/civicrm/civicrm-core/pull/13373.diff",
+ "membership#9, PR#13553 (membership renewal notification fix)": "https://patch-diff.githubusercontent.com/raw/civicrm/civicrm-core/pull/13553.diff",
+ "reporting#8, PR 13653 (thank-you dates on contribution reports)": "https://patch-diff.githubusercontent.com/raw/civicrm/civicrm-core/pull/13653.diff"
+ }
+ },
"patchLevel": {
"drupal/core": "-p2"
},
Note: If you don't use the excellent roundearth/civicrm-composer-plugin but instead use the full manual process to update CiviCRM, you'll need to perform the update steps every time you run composer install with patches.