step1) Add the file you want to modify:: git add vendor/magento/module-customer/Block/CustomerData.php -f
step2) make changes to original file 
step3) git diff vendor/magento/module-customer/Block/CustomerData.php > yourPatchName.patch
step3) you will get the patch file(yourPatchName.patch) in root directory
step4) remove the changes you add to the file(vendor/magento/module-customer/Block/CustomerData.php) manullay by using ctrl+z
step5) apply the patch using :: git apply yourPatchName.patch
you will notice the changes in original file