Magento 2 patch create and apply

Step 1:
 File to changed should be added to the git 
git add -f vendor/dotmailer/dotmailer-magento2-extension/Api/Data/CouponAttributeInterface.php

Step 2:
Then make the changes to the core file

Step 3:
 After making the changes run 
 git diff vendor/dotmailer/dotmailer-magento2-extension/Api/Data/CouponAttributeInterface.php > m2-hotfixes/webapi-fix.patch

This creates the patch in m2-hotfixes

Step4:
  Finally apply the patch 
git apply m2-hotfixes/webapi-fix.patch